upgrade from 16.04 to 18.04, lock screen is stuck

I stumbled into the same issue: Upgrade from 16.04 to 18.04 (on Kubuntu). Screen automatically locked because I didn't use the PC. Entered password, screen is stuck. Switching to the text console - all screens dark. Switching back, mouse courser appears and is responsive, but everything else black.

I then realized:

1. switching to text console with Ctrl+Alt+F1, the text console was not empty but just very dark. So I increased screen brightness on the laptop with the function keys.

2. using top I saw that the upgrade probably still running (heavy load on dpkg).

3. With sudo loginctl unlock-sessions I could unlock the screen savers. I learned this from this answer (full quote for convenience, I used the ):

sudo loginctl unlock-sessions if your system is using systemd. Note that the above will unlock ALL sessions no matter which user is running the screen saver.

If you only want to unlock your own session, just run loginctl unlock-session (no root required because it's your own session).

After that, I changed with Ctrl+Alt+F7 back to KDE and found myself back to the still running session and upgrade process.

I am also still having this issue, but I found a workaround - Ctrl+Alt+F1 brings me to the normal login screen where I can enter my password and unlock the desktop. This method keeps the original session, so your browser, terminal and other apps are where you left them.