It happened to me three times in the last couple of days, after a set of updates.
If I leave my machine unattended for a while so it goes suspended, and I try to log back in.
I hit a key so gdm password prompt comes back.
I type my password, hit enter.
Then there is no change anymore. The login screen stays up infinitely.
The cursor moves around.
Once I left a video paused. When I clicked, the sound of the video came back, but no change on the screen. If I click again, the sound pauses, etc..
So it looks like I'm actually logged in but the screen stopped refreshing.
Ctrl+Alt+F... keys don't do anything.
I can kill off X with Ctrl+Alt+Bkspace (that I had enabled) and then I can log in as usual.
I have no idea what log files etc. to have a look at.
Thank you for taking the time to report this bug and helping to make Ubuntu better. Please execute the following command only once, as it will automatically gather debugging information, in a terminal:
apport-collect 1821534
When reporting bugs in the future please use apport by using 'ubuntu-bug' and the name of the package affected. You can learn more about this functionality at https:/