Thanks for the advice, my tom tom packed up today (Green light on but will not turn on) looked up repairs online quoted £59.95, then read the above posts pressed reset and hey presto worked 1st time.
I purchased two tom tom 910s for my motorcycles of which was not a success at all due to the hard drive failing when accessing when being riden, appears the bumping around and hard drive access at the same time was the cause of the failure. To resolve this failure the reset button had to be used and each time this was done the units came good, Only use them in the car now, have no problems of crashing whilst being used in the car.