TomTom 720 - Does not boot up and a red cross appears?

I haven't used my TomTom 720 for a while, and now that I want to, it just doesn't work. I have had it plugged into my laptop and a PC for a good number of hours and when I switch it on, a red cross appears on screen after a while. It won't connect to my PC or Laptop either so I am unable to use the TomTom Home facility to recover anything - it is just recognised as a mass storage device. This problem occurred before and when I sent it to TomTom, they said that there was nothing wrong with it, returned the device to me and it worked fine. This appears to be the same problem but I don't know how to fix it. Should I be trying to charge the device with the car charger as opposed to the docking port??? any advice would be much appreciated......