If Services can't be activated anymore, they can be restarted by using Registry Editor.
1. Find out the name (or a part of it) of the specific Service (in Service Manager)
2. Start Registry Editor (regedit.exe)
3. Browse to branch HKEY_LOCAL_MACHINE\SYSTEM\ControlSet001\Control\Services
4. Search the specific Service there and open the folder.
5. Set "Start" Property to value 2 (=Automatic Start)
After Windows Reboot the Service will be started! This solution also works for other critical Services if normal access is not possible anymore.