I would guess that the CELL battery has died or you have somehow killed the BIOS and it has reverted to the default. You need to press F2 to re-enter the correct information which will happen again if the battery is dead.
Hi, yeah there is a hard drive connected but it's been in there a good while so maybe that's gone? I was using it and it just shut off and when I tried to turn it back on this happened. If I press f1 it takes me to the bios and f2 takes me to a set up windows screen and when i put In the windows disk I just got an error. It's a Dell dimension e520