This is typically caused by a bad file either on your computer or TomTom. The easiest way to get past this bit of binary constipation is simply to nuke all of the files you have received. That will avoid whatever expiration date issue happens once in a while and start you out with a fresh ephemeris file.
Start with your TomTom. In the root directory, navigate to the ephem folder. Delete all 3 of the files (one .tlv, one .txt and one .ee file) you found there.
Now to your computer. In the My Documents / TomTom / Home / Downloads / Complete / ephemeris folder, you will no doubt find a whole stack of files called QuickGPSfix, QuickGPSfix-1, QuickGPSfix-2, etc. Delete them ALL.
Now have Home update your unit and you'll get a fresh ephemeris file and others should follow on a normal schedule. If this ever happens again, you now know how to break the logjam.