It is good to try to restore the system through a repair CD if you have one. Have you tried to boot into Safe mode? If not, restart your computer and boot from the CD. From there you should have access to restore your OS. Open the Command Prompt in safe mode and run the command:
sfc / scannow
Then press Enter and wait for your OS to be repaired. If you cannot access this menu while you reboot, enter the BIOS settings. There, navigate to the storage menu to verify that your hard drive is as a first boot priority. If not, set it!
Once you have done this please, come over here to tell me what happened.