The problem I am facing started 2 months back. I purchased my rig around 9 months back.
I use my rig primarily for my 3D CG work, as usual I kept my PC for rendering over night and in the morning the fans were running, proxy cooler was running, however I had no display (two monitor setup, both connected to DVI port of GPU). Once I restarted the system, it started working. Then it happened couple of times more. I read couple of posts in tomshardware forums and realized the problem lies with the GPU. Since it never occurred again for few days, I ignored the errors. However, this new issues surfaced; The system completely powered down at random intervals during my work or when running games and wouldn't turn on.
I removed my GPU (Sapphire OS Boost HD 7950) and send it for replacement. While the card was on replacement, I was running the PC on motherboard's native graphics and system never switched off, not once.
Yesterday (took 25 days to replace the card and my system never switched off once during this time) I got a replacement and Sapphire gave me a Vapour X OC Boost HD 7950.
Till now it worked fine, however few of minutes before I wrote this, it occurred again. System completely powered off and everything stopped. Even when I press the power button, it doesn't start. I need to wait for a while or turn on and off my UPS to get the system running again.
I have a strong feeling that the PSU might not be able to give enough power to the PC. As GPU has been replaced, PSU is the next suspect. I could be wrong, and would appreciate it some can tell me what has happened.
My system spec below.
Motherboard :- Gigabyte Z87-D3HP
Processor:- Intel(R) Core(TM) i7-4770 CPU @ 3.40GHz
WD Green 1 TB (1 no.)
(2) Samsung 120 GB SSD (2 no.s)
Graphics Card:- Sapphire Vapour-X HD 7950 OC Boost
Memory Size 3072 MB
Core Clock in MHz 950 MHz
Memory Clock in MHz 1250 MHz
Total Memory Bandwidth in GByte/s 240.0 GByte/s
Sounds like your PSU is starting to die, I assume without the GPU it probably didn't have much of a load to work under so it could keep up but when you got our GPU back it would need more power and it's probably failing out at the higher voltages, especially if it's not turning back on.
Are you getting a boot loop? do the fans spin when you hit the power button but nothing else happens?
Check if theres new drivers for it. Dont just use device manager ether, go to the manufacturers website and look up your device. If you already have the latest driver and there's a beta driver download it and try.