Yesterday suddenly I could not log in to my WIN 8.1 System. PW was ok but the desktop did not load. Instead I was thrown back to the start screen.
Started in safe mode, did some checks (no problems found) and saved newer things. Starting normal again seems to work, the desktop appeared but nothing really worked. Thank's god I had a system backup from March.
Last week my laptop had problem with Windows update and AVG Antivirus. I used System Restore of Windows to turn it back and now it works fine. You may try it.