Hi
When you reboot does it ask you if you want to run Boot Repair?
You can take a look at this.
Startup Repair - Windows 7 Forums
If it will start in safe mode use System Restore to go back to before the problem.
A lot of people have had this problem after a Windows update.
Try starting Explorer.exe manually in the Services tab, after Ctrl, Alt, Delete, and then use system restore to go back before you had the problem.
If you can get it to work update you video drivers.
If none of these work come back and tell us what happened.
Mike