Hi Dave,
Presumably, you've tried reseting the device using the little hole in the base. You could also try restoring a previous version from the 'Back up and Restore' (page 2 of the main menu of Tom Tom Home). If that fails contact Tom Tom direct via their e.mail help. I've always found them extremely helpful and efficient.
howtomendit.com for free repair help, information and advice.
Try fixfault.com to fix satellite navigation.