From Tomtom web site:
Question
My TomTom device won?t turn on. How can I reset my device?
Answer
If you have problems turning on your TomTom device, try resetting it. This is considered a soft reset and will not delete any data.
Tip: Also, try resetting your device if it is behaving abnormally.
If you are able, please connect your TomTom device to it's charger during the reset.
To reset your TomTom device:
1. Charge your device.
Charge your device for at least 2 hours. Make sure the charging light on your device is on (except RIDER which has no charging light).
2. Remove the SD card.
If your device uses an SD card, remove it.
3. Reset your device.
Use a straightened paper clip, or something similar, to push the reset button. For a visual representation of the reset button, see below.
4. Turn your device on.
Press and hold the On/Off button for at least 4 full seconds.
5. Insert the SD card.
If your device uses an SD card, insert the SD card.