First read through the XPS 8700 Owner’s Manual. The reason is that you are going to remove the front bezel (page 21) that houses the power button. Once removed, press the actual button on the chassis itself to see if the system then powers on. If not, follow the power button cable (page 58) to the motherboard and reset the cable. Test the button on the chassis again. Then examine the button on the inside of the front bezel. Does it appear to be working?

Ok, I removed the front bezel but to press the button on the chassis, I'd need to remove the top cover, the problem is that I couldn't remove the top cover...

I don't know why, but the computer turned on right now, so the button is working, but only when it wants to work. I don't want it to continue like that so I want to know what may be causing this problem, I removed all the dust from the front bezel too.

Page 47 = Removing the Top Cover

I would disconnect/reconnect the power button module cabling.

Sure this isn't a power supply problem, rather than a button problem?

Ok, So Today I opened the computer (I removed the front bezel and the top cover). The button was intact, there's seems to be nothing wrong with the power button module, We tried to reconnect the cables and turn on the computer, but the button still didn't work (even on the chassis). It only worked when I pressed it with more strenght. But, when we turned off the computer (and the stabilizer), there was a orange light in the power module (even with the computer turned off).

So, any leads?

If you're talking about an orange LED on the motherboard, it should be on 24/7 when the PC is connected to a working outlet. It means the system is getting "flea" power but that doesn't mean the PSU is working correctly.

There should be a green LED on back of the PC near the power cord connection that should be on when PC is plugged in. If it's not on, press/hold the little button right near that LED and see if the LED comes on and fans spin. And if you let the button go, and everything stops, then it sounds like a power supply issue.

Some times people hold that button and press the regular power button on front of PC. If the PC starts to boot, then it most likely is the power supply...

My Dell XPS 8700 that I purchased in Dec. 2014 would no longer Power on when I pressed the power button on the top cover.  Long story made short - it was the power button itself.

Before I tested it, I tried many of the items in this blog series as well as others, like removing the CMOS button battery, removing different components, like SATA drives, video and other cards, memory modules, etc.  I was not looking forward to replacing the PSU or the motherboard, although that seemed like a next step.  I don't remember who suggested it, but I do remember on person who did replace the PSU and mother board with no success, but finally got it to power on after pressing the power button many times suggested that it might have been the power button itself.  So I tried bypassing the connection wires to the power button and discovered that the 8700 did power on (big pleasant surprise!).  BTW, if you need to access the power button module, download the 86 page users guide from Dell support first.  Getting the top cover off without damaging it is a little tricky unless you know the proper way.  New $29 power button assembly from Dell was a pretty inexpensive solution to the problem in this case.