Did you install a new card in your system that is using a PCI slot?
If you did, look in the documentation from the vendor, as I am sure that is a known issue. If it isn't listed, you have the number for their tech support.
If not, then try this:
Clear the BIOS and then reboor the system. All system boards have a pin to allow you to remove the battery backup power to the BIOS. Make sure you have removed AC power before doing this.
After re-setting the jumper to allow battery power to the BIOS chip (the way it was set originally), power up the system and enter BIOS. Most modern BIOS chips have a setting of "Return to Default (or factory) settings. Choose this option and save. Allow the system to boot normally.
What OS are you using?