causes and things to try to fix it:

--Somtimes can be sorted by an SD card format and a card restore from PC

--is the the TomTom Navigator.EXE within the TomTom folder on your SD card -
[delete the .exe from "my device"]

-- The Map is not compatible with the TomTom version being used

-- The Map is not in the correct folder, normally the My Documents folder

-- The map has not been unlocked correctly