GA-Z77X-UD5H Wil Not Turn On

[SC]

The main PC components related to this problem are:
Gigabyte  GA-Z77X-UD5H motherboard
Intel i5 3570K CPU
XFX PRO 850W PSU

The PC has been working fine for 11 months until yesterday. I switched on the PC at the mains but the PC would not start on pressing the PC power on button. No fans were  running and there were no signs of life. The power LED on the Gigabyte motherboard was not lit. I then reset the XFX PSU by toggling the PSU switch on/off quickly. After about 1 minute, the power LED lit on the motherboard and the PC booted OK. After turning the PC off and switching off at the mains, the same thing happened again. I had to reset the PSU to power up the PC. I then noticed I couldn't enter the BIOS screen and flashed the backup BIOS to the main BIOS. So far things are back to normal!

Can you explain what has been going on here? I assume the BIOS has been corrupted which prevented the motherboard sending the correct signal to the PSU to power up and somehow putting the PSU in a fault condition requiring a PSU reset??
How does the XFX PSU fault detection logic operate and what conditions would put the XFX supply into a fault state requiring a PSU reset?

[ac]

A power surge will cause this problem, where your bios can be corrupted during an outage. It is rare but it can happen. That is why some people use UPS systems. When you booted up the computer, CMOS detected inconsistent bios from Bios A and Bios B. Basically the motherboard detected a check sum parity error. This causes the system to shut down. When you reverted to the backup bios, that fixed the problem.

A power surge is the likely culprit. Sometime this motherboard can tell you there is a problem and offer to copy bios B to bios A. And sometimes
the corrupted firmware is so messed up, you have to manually fix the problem.