Try entering the BIOS setup and choose to "Reset BIOS Settings" to their factory default levels.
The exact menu option in your motherboard's BIOS may differ but look for phrases like reset to default, factory default, clear BIOS, load setup defaults, etc. This option is usually located at near the bottom of your BIOS options.(this would clear any overclock settings)
Might also be helpful to have a look at your motherboard manual for instructions on how to restore or reinstall BIOS settings if necessary.
Restart it a couple times if you want to be sure. It should be fixed though, sometimes things like this happen for no apparent reason, it probably wont happen ever again though.