You can try the restore option if it is activated on your desktop when you reboot. Not all computers have this.

If not then you will have to use the safe mode. Restart the computer and toggle the F8 key until the start page comes up with the different starting options. There are two things that you can try...
1) select to start in the last good known configuration

2) If that doesn't work then you must start in safe mode option and you will need the missing file to replace the one that is missing.