" ... all other parts power on with no issues. My only problem is I can't turn my PC on with the alien head power button"

So we can take at face value your system does power up & works, you perhaps even used your 51 to type your help post with? If so, what is your new means for powering the system up, the rear PSU built-in self-test button, or some other way?

I no longer have my R2 case, & while I've browsed the forum regularly since 2014, it's possible a 'front power on button failed to turn system on' may have come up once if that, which is to say that no ready-made reply comes to mind like 'oh yeah, that happens, just get a new button', which is to then say it's likely I could pass on the wiring scheme involved & you'd take a voltmeter & perhaps acquire a shorting-tool, attempt to diagnose the power-on wiring & panel headers etc

https:// i.postimg.cc/Fzkkkndx/h6hk9.jpg (copy/fix/paste link to see it)

A part like 8248k Logo Board is where the physical power-on button resides; it sends a cable over to FIO board (photo), plugs in etc, where simple power +/- traces (passively) wired (on traces) direct to the FIO front panel header (shown, photo). FIO then sends a cable to the mthrbrd front panel header so mthrbrd turns on via PSU. Pretty simple

When system plugged into wall, mthrbrd sends voltage over PWSW (+) to FIO PWSW (+) to LOGO board Pwr-On button; when u depress AlienHead the circuit closes (momentarily) & the voltage redirects back to mthrbrd PWSW (-) --> mthrbrd sends signal to PSU to turn itself on --> system turns on

I'd likely try to jump (short) the FIO PWSW (PWR ON Switch) pins (called a 'jump start'), see if system turns on. If not, pull your other side panel off & attempt the same but this time do it at the identical mthrbrd front panel (far right middle of mthrbrd). If system didn't turn on by this valid jump start method, report back, it's either the LOGO board button, the PSU or the mthrbrd at fault (= easy fix or hard replacement of part) 

Voltmeter suggested for further tests (probe PWSW +/- for volts, check for continuity-OHMs when AlienHead depressed etc > if volts present, if continuity-ohms when depressed then power-on button is ok/works & issue is elsewhere etc)

I'm using my phone. If I could get my PC turned on, I could set it to turn on with my keyboard in BIOS but I can't even start it. The PSU test button doesn't actually start the computer, it turns on the case effects and fans and everything but no response on the monitor. 

How do I short it? That sounds like something that will blow up in my face.

I no longer have a Dell-branded PSU here & when I did have one I never tried the rear button to see what'd happen

One test is to unplug monitor from wall for a minute then plug back in, see if black screen issue goes away; other is to try a different monitor

If still no video, & if you have access to the photo I posted of the front panel (power-on) header which shows the two PWSW (PowerSwitch) pins - were it me - I'd remove the Hard Drive (right) Side Panel & access the FIO board, & with a paperclip or similar metal tool (tweezers) or even a jumper wire, 'short' the PWSW pins together momentarily, 'jump' the pins in an attempt to manually 'jump start' system on

Alternative is to open the Mthrbrd (left) Side Panel, find the front panel header (far right middle of board at the very edge), back the connector off, find the raw PWSW +/- pins & short / jump them as above, in this case a flat screw driver tip can be used (driver tip can not be used on FIO header above, from earlier)

See uTube, search = jump start motherboard without power button

If system still will not turn on manually (jump started) after shorting PWSW +/- pins together, issue isn't the power-on button & other steps are needed to diagnose the no power-up issue (remove &/or replace onboard coin-cell battery for instance); doing the recommended steps for the built-in self-test etc

I'm going to try this after work. Is there any way to open up the button to fix it or replace the spring if that's the problem?

One other question: is one of these the circuit I'm looking for?

 Sorry for all the questions, I'm pretty nervous about unplugging stuff in my PC because I don't have much experience doing this. Asking questions is the best way I know to get comfortable trying something new. 

Your photo hasn't published yet, but mine did of the FIO board which sits in Hard Drive bays (right) side of case. I posted a link of the 8248K Logo board, which sits behind the Alienhead, top right wall of (left) side of case, the link shows the (micro, red) on button. There is no spring. If it turned out the button / switch is bad (rare, but could happen) then you'd replace the Logo board

I suppose you could remove / back out your Logo board today & press the micro-button. If PC didn't start, you then need to find out why

I can't see (yet) the circuit photo of yours. Until I see it, I explained the on-off wiring scheme in the prior post. A voltmeter & a shorting tool (or jumper wire) is suggested. There are 3 places to attempt on-off:

• Logo board button
• FIO FP header
• Mthrbrd FP header

While u can test the Logo & FIO, I'd cut to the chase & begin at the true source of on-off 'circuit' by finding the motherboard FrontPanel header (FP)(I call it a 'rainbow' header, far right middle edge of mthrbrd above SATA & above USB2.0 header), it's the same rainbow header as found on FIO board, they're twins, see my pic. It should be the case that within a minute you've found & backed the mthrbrd FP rainbow header connector off - carefully touch (short / jump) the PWSW +/- poles together & try to jump start system on --> it either will or won't turn on

If it can jump start on directly through the mthrbrd, it means the on-off parts downstream are suspect (FIO or Logo)

If it can't jump start on, it means FIO / Logo most likely ok, & the fault is in the mthrbrd, PSU, related hardware or your wiring (like a short, ie maybe the PSU is in short-circuit-protection mode & won't start until it's fixed, for safety etc). If it won't start, I'd do the PSU built-in self-test, follow the guide & report back

Some PC owners keep spare parts like PSU, memory, GPU, onboard coin-cell battery, hard drive, even mthrbrd/CPU in order to test with. For those who don't, 'is the PSU bad?', 'is the mthrbrd bad?', may be an answer that forum members & the afflicted owner can't get answered until a spare is purchased & dropped in to find out. Remote diagnosis isn't easy

Back off the FP connector & attempt to jump start at the mthrbrd FP header 1st (the source), then consider, I suppose as an after thought, push FP connector back onto header then remove / back out the Logo board & try to press the physical button. If PC didn't start, look at the 3v coin-cell battery (remove, test for volts; here, if mine read less than 2.65v then I replace it). After battery is back in, attempt to start through Alienhead. If no joy then continue to troubleshoot (PSU BIST etc) & find out why

Typically we might remove GPU(s), disconnect Hard drive(s), remove all but a single stick of memory, only have keyboard mouse, try to power up, what happens? No joy?, power down, remove single memory stick, power up --> did mthrbrd make a beep? If so, good, if not, bad sign ... ... ...

Where I'm at is, like u, trying to find out if the on-off button still works or not (generally we'd by-pass the button & jump start the mthrbrd on at the source of on-off: the FP header)

I have your mthrbrd, mine's below inside of an R1 case (https://  i.postimg.cc/QCzQwmxy/PTDC0004.jpg), if I want to turn it on I can back off the FP connector, expose the FP header & jump start at the FP header PWSW +/- (flat screw driver etc) within 30 seconds or so (as seen on uTube link / search, prior post). The FP header is identical to FP header shown in FIO pic, (prior post)

I'm looking at my motherboard and have successfully identified two inputs labelled fan 1 and a big black rectangular connector in between them. There's also a third fan 1 and an audio near the rear of the case on my motherboard. On the opposite side still near the front, are a sensor 1 and 2 connection a PCIE/PWR1 and...a fourth fan1 PCI. And I'm very confused. 

If u copy/paste/fix the link to my photo u can see it (Https://)   i.postimg.cc/QCzQwmxy/PTDC0004.jpg

On far right edge of board are x3 rows of black SATA ports (where your HDD & ODD data cables plug in), right above those is USB2.0 header, right above that is the Front Panel header you're looking for

I think I see it. Its between my two video cards and I can't really fit my fingers in to reach it. I tried using tweezers but they're too short too.

SATA ports like where my video cards are hooked up? Is it between the two or above them? I don't see a USB header.

I'm trying to use your picture but it doesn't look like what I'm looking at here.