Can you boot into Safe mode?
The first that comes up into my mind is to restore the system through a repair CD if you have one.
The second thing is to restart your computer, go after pressing the appropriate F key to navigate to the Windows menu. 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!