Strange Garmin problem

[Dr Jekyll]

When sitting stationary in a motorway jam I spent a few minutes exploring the settings on my Garmin satnav to pass the time. Didn't change anything as far as I can tell, but since then it's stopped navigating. It looks normal, the speed reading works, but the position doesn't alter and the ETA goes up by a minute every minute. If I try to start a new route, even to the original destination, it finds my position and gives a realistic ETA. But then sticks to that location with the ETA going up despite the speed showing correctly.
Any suggestions?