Chief - be honest
- have you fiddled with it (the 'puter) lately?
Sounds as if it is not seeing your boot drive for some reason. If you are familiar with access to the bios settings on boot up ("Press 'DEL' "it should say) have a look at the drives it can see versus the drives you expect if to see. It could be that the drive cable has been dislodged, drive is knackered, boot sector is corrupt etc etc - delete where applicable!
Assuming that is ok, I assume you have a 'boot' floppy disc or CDROM? Can you boot from that?