Thanks, YanaYana. Yes, it is working in general. I'm trying to reinstall Parallels Tools but I can't get the terminal to launch: the menu bar is missing from the top of the screen and I can't find the drop-down that contains Terminal by working blind.
I've also tried Alt+F2, and this isn't launching Terminal either. The function keys are working - F1 launches Ubuntu help - and pressing Alt+F2 sometimes makes the "black hole" change colour from black to green, or vice versa.
I haven't got any data associated with this Ubuntu installation so I'm okay with deleting it and starting from scratch, if that's easier?