Screen went black
Looks like you accidentally switched to the text console, that does no harm at all as long as you know how to return to your desktop. Did you accidentally hit Ctrl+Alt+F4? In that case you can return to your desktop hitting Ctrl+Alt+F7.
There are usually 6 text consoles (F1 - F6) but most users aren't even aware of them. They're extremely useful for troubleshooting, but otherwise barely used these years. I think the main use is in servers with usually no GUI (graphical user interface) available.
The shortcut including the F7 key didn’t work for me in Ubuntu 18.04. Instead, Ctrl+Alt+F1 worked to get back.