You can search for this bug and it seems like it may be related to ecryptfs and is many years old.
The symptoms are that you return to the computer and the screensaver was active or the screen was asleep/black and it doesn't seem to come back. But you check by SSH the computer is running fine and are frustrated you'll lose your running programs and have to reboot.
There is a simple solution:
Ctrl + Alt + F1
Ctrl +Alt + F8
Ba........