Are you able to get into the BIOS?
If so, check to see the boot order and what boot devices are enabled.
I'm going to assume your OS is on an internal HDD...does it show in the BIOS as a bootable option? Is it sequenced as the first bootable device? What type of HDD is it, IDE, SATA? If IDE, have you checked the master and slave jumpers?
Lastly, do you know if the OS had any issues...any system files that became corrupt?