I had the same problem.
My 720 worked fine right out of the box, but once I did an upgrade, it turned into a piece of plastic.
I notified customer service, but after 2 days and hearing nothing, I returned the unit to CircuitCity for another.
SAME PROBLEM!
The home2 backup is not exactly what I would call "robust".
The update corrupted the operating system.
Delete it from the home2 page (ignore the warnings) and update again.
When you bakup, use Widows and backup everything.
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Today, I received this from "support"
Thank you for contacting TomTom Customer Support with regards to your device
It sounds as if the update has corrupted the application
Connect the device to the PC .Close Home ,Open My Computer .the Tomtom will be recognised as a mass storage device
Explore it and delete all the loose files that are not in yellow folders .
In Home v2.0
Preferences/Folder preferences/Downloads/Browse and delete the downloads folder
Exit Home and then open it again ( this will create a new downloads folder)
Log-in again and click on download your updates
In Home reinstall the application update and this will be rebuild the operating system on the device .
With Best Regards
The TomTom Customer Support Team