Screen Saver Stuck On, How To Escape?
I encountered this problem for the last two days on Catalina on my iMac. I tried some suggested methods, but lacked an Apple keyboard, and didn't restart or use SSH.
What eventually worked for me, I didn't find elsewhere:

Ctrl+Cmd+Q
Esc
login

Hitting Esc makes the screen go black, which I guess cancels the screensaver.