Have you made any changes to your computer prior to the issue?
Method 1: Run SFC scan. System File Checker (sfc) scan will make Windows replace corrupt or missing system files on your computer.
Steps to perform sfc scan:
a. Click Start, click All Programs, click Accessories, right-click Command Prompt, and select Run as Administrator.
b. Click Continue or supply Administrator credentials if prompted.
c. In the Command Prompt window type the following, and press Enter:
Refer this KB article for more information: http://support.microsoft.com/kb/929833