The simplest thing would be to enter the BIOS, press Del as it's booting up.
Choose Optimal Defaults, then save and Exit the BIOS.
This should reset the PC to non-overclocked settings
First, load your BIOS setup and restore it's default settings, then if you used a program to OC it, restore it's default settings as well. or manually roll the clockspeed on the CPU back to normal. that should fix the problem.