Have got a repair CD of your system? If you do, try booting from it to see if this will repair your operating system boot files.
If you don't, that reboot in Safe mode your PC, if you work with Windows and open the command prompt console.
There you have to type the command: sfc/ scannow
Once you have done that, check your hard drive for errors by entering the following command: chkdsk /r
After that restart normally.