Sounds like a hardware fault. Try another USB cable (you don't need the Tomtom dock, any USB cable will work). If that doesn't work, see if dirt is lodged somewhere in the USB jack.
Otherwise, the 720 can run everything off of an SD card. You could get an SD card, use winrar to extract the contents of
this file onto it, stick it in the Tomtom, and turn the Tomtom on. Once powered on the Tomtom will add more files onto the SD to identify itself. Then you can insert the SD into a card reader and update it as if you were updating a plugged in Tomtom.