Dell XPS 8940 Desktop Turns on but no booting and mouse and keyboards off

What color is the power button when it won't boot?

Sure the replacement PSU is compatible, and its connections to the motherboard are correctly reconnected?

Have you tried replacing the motherboard battery?  Unplug PC, and press/hold power button for ~15 sec. Remove old battery, press/hold power button for ~30 sec than install a fresh one. 

Well, I have to ask, WHAT shows on the Monitor? Is it the Dell Logo? Is the Monitor connected? Or does the Monitor still stay black?

Have you checked the Monitor Input on its internal Menu?

@ispalten​ it shows nothing, the computer just turns on the fan, and the power button lights, but not booting up, usb no power, nothing on screen, doesn't boot up at all, only turns on Fans and Power button lights, nothing else, i'll upload a video later.

Monitor input is all good, as long that the mouse and keyboards turns on the lights i know that pc can boot up, but after that if i turn off the pc the next day the pc wont work at all until i unplug it and plug it back again the power cord, and keep trying until it boots up

NOTE: i have bought 2 additional power supply, and is the same problem, so Power supply is perfect, and tried to plug it into another outlet to see if was the outlet, and is not.

@RoHe​ yes, tried to do that and sometimes works but after i turn off the pc the same problem occur.... so yeah... tried to do so many things and haven't have found any solution yet

You didn't say what color power button is when it won't boot.

Sure sounds like a power issue. So if multiple compatible PSUs don't solve the problem, it's possibly a motherboard issue.

BTW: Have you tried replacing the power cord from rear of PC to wall outlet? Are you sure the fresh battery is being correctly seated in its socket? Have you examined the battery socket for potential damage?

So here is the video of what happen with my pc, if someone has any solution, please let me know

@Dandy Ng  Vid doesn't play, at least not for me...

@Dandy Ng​ The video doesn't work for me either, can you let us know how you attached it?

Problem IS the video I suspect, note the TIME is 0/0, in other words, there is no content.

sorry guys, here is the video that i uploaded because looks like isn't working here
https://youtu.be/6weF9nQVt9c?si=cYfAzMsoBFACuVvq

@Dandy Ng​ 

OK, see that.

I see tape on the left top side of the case? What is that for?

Not clear if the lower front fan is running?

Did you try connecting the Monitor to the on-board Intel card? Wondering if the video card failed? Just something to try to rule out the card. That assumes the on-board will work and not disabled by BIOS?

There is a 'disk access' light as well on the back of the unit, does that blink?

@Dandy Ng - What are the 2 devices plugged into front of PC?  Have you tried disconnecting them?  If they're the mouse and keyboard, they should be connected to the 2 USB2 ports on rear of PC.

Exactly what was that message on the screen in the vid? Not there long enough and too blurry if I pause the vid to read.

Why do you have to hold the power button for so long before PC shuts off?  Don't have this PC model, but don't think it should take that long to power off.

You could strip it down to bare essentials:
Remove video card, disconnect all internal drives from motherboard, except boot drive. If boot drive is NVME SSD, carefully reseat it in the M.2 slot and reinstall the retaining screw.  If the boot drive is a SATA HDD (or SATA SSD), make sure it's connected to the blue SATA port.  (See motherboard layout here.)

Remove WiFi card; remove all RAM modules except the one in 2nd slot from CPU; disconnect internal card reader, etc.  Then check all connections to motherboard from PSU, power button and front panel. 

Try booting with only mouse and keyboard connected to USB2 ports on rear of PC, and monitor connected to an onboard Intel Graphics port. If it gives an error message, copy the entire message and post it here.

@RoHe​ 

Why do you have to hold the power button for so long before PC shuts off? 

Ron, I didn't comment on that, I suspect he thought he'd be clearing the BIOS, but also some other stuff, but that had to be done with the power off. It is what is supposed to drain FLEA power that some capacitor's might be holding and preventing booting. Also part of that was to remove the BIOS battery and holding the button too according to some Internet posts.

I don't think Flea Power, and it IS called out in the 8940 Manual:

But even if that was what @Dandy Ng was trying do, the fans kept spinning for a lot longer than I would have expected...

Ever find the solution? Mine is doing this now, too.