Sorry, I should have been more specific - no, it just hangs - I am unable to log on hence my concern. I could do a fresh install but then I would loose everything on the computer. Thanks for the advice John, but I am an illiterate and the logs, even if I could work out how to boot into safe mode, would be meaningless. With regard to the Graphics card however, the computer has always run fine with all the previous Ubuntu releases that I have used
I'd suggest logging in via the console (hold ctrl and alt and press f1) as your user, then running sudo aptitude update sudo aptitude dist-upgrade and making sure it has installed all the packages it wants. it sounds like it might have failed to finish the upgrade. If you are prompted for a password when running a command starting with sudo, enter your users password.