We had this a couple of weeks ago, it blocked OH's account and the Admin a/c so I couldn't do anything as it wouldn't accept instructions from me.
Are you able to access start on the computer? If so go into System Restore and pick a date prior to when you could last log on as it it could be something that has recently downloaded thats now blocking you. Accessed through Control Panel, System Maintenance, Back and Restore then use System Restore to fix problems and then follow instructions.
Have to admit this didn't work for us and in the end had to completely go back to restoring factory settings, having backed up all my documents photos etc. This I had to do through Recovery Manager.
I'm not that technical on IT, so may have gone a bit OTT but it was the only thing that worked. Perhaps someone who is more technical than me might be able to suggest a different route.