I had this problem, but the USA and Canada Maps were corrupted in some way and they could not be re-installed from TomTom Home. It was as if my maps were deleted entirely
I called the TomTom customer service and they were surprisingly helpful. They allowed me to download a new copy of the USA and Canada Maps - I was afraid they would make me purchase them, but they did not.
So if this has happened to you, try calling the customer service.
The_Juggler, June 2008