Okay. I think I found out the actual way to fix it:
Open the "Session and Startup" app. It's under the Z icon in the bottom left corner, and under Settings.
click on the far right tab (right now it's called "Advanced", but I think it used to be called 'saved sessions' or something).
there was a saved session for startup. I reset it to default.
A new question: is there a way to remove the option to "Save session for future logins" permanently? (it appears in the shut down, reset, logout, etc window). I think this caused the problem (it happened in Zorin Lite 15 too), and I only clicked it by accident (and also turned it on when I pressed alt+s when I meant to press alt+d), and I don't even know what it's supposed to do.