I've twice had the following problem (running Linux Mint 20 (xfce) on AMD 64 (8GB)):
While using shortcuts in apps for example Ctrl+F5 in Firefox, my fingers slipped (fast) and I accidentally hit some shortcut I didn't notice (because of speed) and suddenly the system was in Terminal mode asking for login, and there was no way of getting out if it.
There must be a shurtcut to start and to close this terminal, that overtakes the screen and locks you out. I've tried search engines but find nothing.