Sometimes these problems occur if you have a Screen Capture program running - it locks the screen.
On some occasions, the programs we are really supposed to lean on, Antivirus etc. cause these issues.
1. Turn off / uninstall any Screen Capture, and check, through restart - if it helps, fine.
2. Turn off ALL security programs, restart, and check - if you have Windows' latest updates, you'll be completely safe for several hours BUT you must remember to turn them on afterwards.
If you have several security programs, they may collide. Do give a notice of what you have in use.