try shutting down, turning off the PSU switch, and resetting the BIOS. That can be done by removing the CMOS battery and shorting the CLR RTC pins on the board for a few seconds. Replace battery, power up, and try to boot. You will then have to enter BIOS setup and change any parameters you had modified from default if any were made. If you are unsure of how to do it, give us your board's make/model.