They sometimes take about 30 seconds to find enough satellites to work, so maybe give it a minute to work out where it's at. You can often speed this up by connecting it to your computer and it'll update their locations for quicker startup (needs to be done once a week).
Is this failing to find satellites when in the car? Some cars have thermo-reflective windscreens and it can cause problems with stopping the GPS signal. I've got a TT510 and a Peugeot 206 with one of those windscreens and it works, but it seems to depend on the car.
You can find that it'll lose the signals if you're surrounded by tall buildings, or in a large building like flats due to the metalwork. Try leaving it outside for a minute and see what it finds.
Assuming the software is the same as the 510, when it starts up, click the bottom right of the screen (when displaying a map) and it should tell you how many satellites it can find. From memory that needs to be at least 3, but outside it should find at least one.
If you find it's the car causing the problems the only answer I'm aware of is to buy an external antenna from Tom Tom under accessories on their web site. They cost about �40, although I have no idea how you get it from the outside to the inside of the car.