First, try resetting the BIOS as follows:
Power down; unplug your computer from the wall socket; press the power switch for 5 seconds (clears the capacitors); remove the BIOS battery; move the Clear CMOS jumper per your computer manual book instructions, usually from one set of pins to another, wait 10 seconds, then move back into original position. Replace the battery and try to boot. Now...if your board does not have a clear jumper, then skip that step. Just remove the battery with the power cord disconnected, wait 5 minutes, then replace it. Try to boot.
Have you tried booting into Safe Mode? If you can, do a system restore to a restore date just before this happened.
Post back with results.