Don't boot-up anymore

How can I get into the bios (Ctrl-S?) to set the hard disk as the 1st boot disk? With ctrl-s I get into the bios but I can't set up harddisk/cd/floppy or anything, indicates that no other settings are possible. Has stood still for a while and starts now suddenly no longer on, possibly back-up battery on the motherboard empty causing the setting to be changed? Who can help me?