Xubuntu 12.04, mouse cursor disappears
A temporary work around is to use Ctrl+Alt+F1 to go into a virtual terminal and then Ctrl+Alt+F7 to go back to the GUI. The cursor then comes back.
I'm having the same problem with Ubuntu 20.04 as of about a week ago. I don't know what's causing it or how to fix it but when I type Alt-Tab I get the cursor back. Try that as a temporary work-around.