Alan - my Tom Tom did this a couple of times last week - leaving me temporarily 'lost' in an unfamiliar city. I tried resetting it a few times - paper clip in a hole in the back of the unit- but that didn't seem to work. In the end I left it for 10 or so minutes 'Waiting for a signal' and it eventually found the signal and 'rescued' me. My sister in law had a similar problem on a Garmin. We decided that it wasn't a sat nav problem but rather a satellite one. Hope this helps.