No log-in window after wake up, keyboard input results in screen flashing

[Darvari]

Something weird just happened to my mini (the latest model): when I tried to wake it up, the screens (have two) were turned on, but there was no picture, i could only see the pointer of the mouse - no login screen. Whenever I was hitting a keyboard key, the screen was flashing briefly.

Finally I had to force a shutdown. Anything like this happened to anybody? What would cause this?

Thanks