Try rebooting into Safe Mode, then start System Restore to wind back to when it worked. Hopefully you can figure out what happened to cause this.
To easily start System Restore in Safe (deprecated) mode Press Windows key + R and type rstrui.exe and hit enter. Select from the menu displayed.
If the issue still happens in Safe Mode, try running System File Checker.
To do so:
1) Click Start, type "cmd"
2) Right-click, Run As Administrator, Yes to the UAC dialog if it appears.
3) Type "sfc /scannow" in the Command Prompt which appears.
If any issues are found, they will attempt to be repaired.