It may be that your motherboard battery has died, in which case it is just a matter of replacing it. It should be a CR2032 button cell. If on the other hand it isn't that I would try disconnecting the mains cable and removing the battery and then resetting the CMOS. Leave the battery out and change the jumper for at least 10 seconds and then replace the jumper to its default position and refit the battery. You can then boot the machine and redo your bios settings.
