That's impossible. Since you didn't tell anything about the what and why ("it does not start" doesn't tell anything about what does happen instead ) I can give just general fixes:
1. System file checker (sfc /scannow from the command prompt)
2. System restore back to when it was OK
3. Back to your latest image backup
4. Reinstall Windows

With more details, also about what went wrong, their might be other suggestions.