Try leaving the car stationary with the key card in half way.
It might take 20 min before it has tracked the satellites.
You can also go into settings and diagnosis with the code 4112. In there you can tell the sat nav where in the world your car is to help speed it up a bit and also view the satellite signal and number of satellites found.
This problem usually happens when the battery is disconnected or goes flat.
Always wait 15 min after turning off the ignition before removing the battery.
If all that doesn't work, you can get a new GPS aerial on ebay for about £6-12.
You might find that as well as the satellite problem, the clock resets too. I think that the time is updated by the satellites. Not sure yet.
Anyway hope this is of some help to someone.
Nik
January 2011