You most probably have altered a setting and the bios does not like it.
You have two options.
1) is to go through and check each setting, not as bad as it sounds. But you would need to know what hardware you have. e.g. hard drives, optical drives, exact spec of cpu, memory and if you have a floppy drive.
2) set the bios to save defaults. This would reset all, but it may have your system running slower than it should.
Of course it could be that you battery is flat. Is the date and time accurate in the bios?