Windows 7 Windows Defender

Windows Defender stopped working. I can't run it at all. When I push the button to scan, nothing happens. When I run the executable (MSASCui.exe) in the Windows 7 program files folder it says a problem caused this programs service to stop. When I click the start now button, it doesn't start and doesn't work. It just times out and give me an error. How do I fix this?