This has happened to me a couple of times, ironically shortly after updating through My TomTom. I normally just press the center button at the bottom of the screen (where it displays the GPS locating message) and it is able to find my location then.
I've noticed this only happens to me when I put in my location/address when I am in the parking garage (and therefore don't have a GPS signal) and then drive out into the open. If I wait until I leave the garage and then enter my address, I don't have an issue with GPS fixation.