I have an Area 51 that I recently swapped out the psu.  A Corsair RX750m.  Has been working without a hitch for a week or so.  Suddenly while playing Wolfenstien the computer shut down, black screen, no power. Pushing the power button wielded no response, no click or fan spin.  Tried unplugging almost everything but no change. Swapped back to the stock psu, no change. Pulled ram and reinstalled and again nothing. Every now and then I get a quick led flicker or fan spin but it instantly goes dark.  I have a feeling it's the motherboard but don't have a spare to test. Before this happened every few weeks the computer would shut down ant there would be a loud beeping from motherboard.  Its a repeating three beep pulse. Any ideas from here on testing? 

 

Stock i7 5820k

EVGA GTX 1080 founders

Corsair RX750m

16gb ram, half stock, half crucial.

Stock Samsung 256gb SSD.

Stock motherboard.