This diagnostic chart may help. If the power button is Amber and you see the 1-2-3 light sequence, that is a coin cell failure. Replace the CMOS battery (coin cell) with a brand new recently purchased battery (CR2032). If you see a normal power button light colour then it could be a drive problem. Check the drive cables and make sure the boot sequence is correct in the BIOS.
So my PC always worked flawlessly until some days ago i came home from work and pressed the power button, i got no response, fan doesn't spin, no light, just a total blackout. I tried pressing the BIST button, the fans started spinning until i released the button.  I unplugged all the cables and disconnected my RAMs, disconnected the CMOS battery and tried powering on after a few minutes. Didn't solve the problem except the fans started spinning and stopping on their own(POWER BUTTON STILL NOT RESPONSIVE). I slept with the power cable on and the fans spinning and stopping like that, and woke up in the morning to find it spinning and stopping, blinking light 1-3-2 at every single spin. Sometimes the computer powers on (on its own) and works perfectly, and suddenly after a few hours it powers down and return to the eternal loop. Please help  Core i5 2400 Processor 20GB RAM 8+8+4 2TB HDD  The computer is not modified at all.
Dell Optiplex 790 won't power on