I overclocked my CPU and RAM using auto TunerEVO in Asus AI Suite II utility, set it on Extreme mode, which gave me 4.550Ghz. It was working fine for the whole week, I was playing lots of games no problem. Today I played Witcher 2, and after 20 min of gameplay screen and sound freezed. I had to turn my PC off and then I turned it back on. It gave me an error "Overclocking failed: Press F1 to enter SETUP", I didn't do anything in BIOS, just loaded Windows and saw that CPU now is running at 1.2Ghz. I set auto tuner again but at Fast mode at 4.25Ghz instead of Extreme.
What happened? Should I keep it at fast settings and see how it will go on or set it all back to factory clock?
Your fine. All that happened was that the system failed to initialize during POST due to your OC settings. If it worked fine for a week it was probably a fluke. I'd try it again, note your voltage settings for the RAM and CPU Core and if it fails again in the same way then you can up your voltage to a bit to compensate (I think 1.35V for CPU and 1.65V for RAM still hold as general rule for Sandy Bridge E).
But why did it freeze during the game? Something cannot handle high CPU frequencies because of a low voltage?
Right. The voltage was probably a bit low for the load it was under and it crashed. I'm not familiar with Asus's software, so I can't say how it achieves the overclock and what settings it turns on/off in order to get there so I can't offer any solid advice on what other BIOS settings should be changed to get things more stable. I can tell you that VDroop should be set to low/minimum though and PLL overvolt should probably be on with clocks that high.