Just had this problem today. Found that the following fixed the problem:
On bootup press F8
Select the Repair option (already highlighted. I missed it to start with thinking it was a heading).
On the program that loads choose the option to restore.
On the list of restore points choose one from a few days earlier when you computer was last working. (My last restore point had been created the day before the problem and was created due to a Windows update so assume this must have caused the problem.)
Once the restore has finished you should be able to login as normal.