It seems that occasionally GNOME can go a little screwy and its fancy mouse pointer plugin can result in an invisible mouse pointer. Which makes doing anything just a little bit tricky.
If you can open a terminal, enter the command :-
gsettings set org.gnome.settings-daemon.plugins.cursor active false
And all should be well. At least until it decides to turn itself back on again (so make a note of this fix!).
