Most TomTom models are restarted by pressing and holding the On/Off button until the device restarts. Typically this takes 15 to 20 seconds, but may complete faster or slower.
For some TomTom models, you need a straightened paper clip. Using the clip, press and hold the built-in Reset button for 20 seconds and then press and hold the On/Off button for five seconds. If your device uses an SD card, remove the card before pressing the reset button and re-insert the card after starting it back up.