Mouse is stuck in drag mode (hand icon). Cannot click anything other than the top bar. Restarting doesnt fix it!
In my case I knew which program had caused the mouse cursor to 'freeze up'. I was using Archive Manager to drag and drop an image out of a zip file when I got the hand icon. Tried the other answers here without luck.
What worked for me was Alt-Tabbing to activate the application then press Alt-F4 to close the program window. Once the window closed, my mouse cursor returned to normal.
I fixed this myself by changing the setting in the terminal.
Open a new terminal with CTRL + Alt + T and enter:
gsettings set org.gnome.desktop.wm.preferences mouse-button-modifier "'<Alt>'"