Try to unplug the mouse and put it back in.

If that does not work then open a terminal and upgrade from there. You may
have a partial upgrade where it upgraded some things and not
others. Sometimes timing plays a role. When you hit the upgrade button it
may not have all of the updates that you need to make it complete.

sudo apt-get update
sudo apt-get upgrade

I would try unplugging once again if it does not show itself. Then I would
try a re-boot. I would also try a different mouse.