Hi,
1. Have you made any changes to your computer prior to this issue?
I would suggest you to follow the below steps and see if it works fine:
Step 1
Follow the steps from the below article
How to use the System File Checker tool to troubleshoot missing or corrupted system files on Windows Vista or on Windows 7
http://support.microsoft.com/kb/929833
Step 2
If the above steps don’t work perform system restore
System Restore
http://windows.microsoft.com/en-US/windows7/products/features/system-restore