Well, choose 1 if you bought your PC built (and haven't changed the motherboard), 2 if you built the PC or have changed out the Mobo.
1: go to your PC manufacturers website and go to their support/download section and find the BIOS there, possibly update your drivers while you are at it, that can fix allot, even if you have the current version if they were somehow damaged this should help.
2: go to your motherboard manufacture, and download the BIOS, and install.
3: tell me your PC make or MoBo make and i'll find the BIOS if 1 and 2 don't work out for you.