In the past weeks I have come back to my Ubuntu 16.04 installation several times to find myself staring at a blank screen with only a cursor on it. In those cases I was expecting the lock screen, which is indeed what is shown most of the times when I wake the system up (monitor is turned off after lock screen has been shown for a while).
I can switch to the first console terminal using Ctrl+Alt+F1, but I can't find what's wrong. The process list seems fine, there no process hogging the CPU and restarting the X server or Gnome of course leads to loss of all current state.
The system does not suspend or hibernate.