@All:
If Rich is unable to see the contents of the internal memory of the 720 in My Computer, Home isn't going to be able to see it, either. So until we sort that out, using Home to manipulate the device is a non-starter.
@Rich
We're going to force Home to reload your firmware if possible. That's a little tedious, but is the easy part. Let's see if we can get this done without having you pay for another map.
dhn is 100% on the right track about the maps. If you do have a backup, let's see how old it is. Your "From Device" is where you have used Home to back up your unit, and that North_America_2GB.zip file is EXACTLY what you need. Open it up with your favorite tool and see what kind of dates the files have internally. If it's recent enough, you can use those.
Now ... moving on to restoring your unit.
NOTHING on your unit's internal memory is visible, and you've got at least some map stashed on your PC, so let's use Windows to format your unit. If that is successful, this will be easy. If your internal memory is no longer functional, it's a bit messier since we'll need to install the entire works on an SD card. FWIW, if the unit firmware is found on an installed SD card, it will boot from that and ignore internal memory, so even if internal is bad, the device can be used.
Remove the SD card for now. Connect up to your PC. If Home should fire up, just close it down.
Use Windows to format your unit, FAT32, and NOT Quick Format. Once you have done this,
use the systray icon to properly disconnect your device. Go ahead and turn it off.
Make sure you have Home2 installed. I can't tell if you've done this or not, only that you had previously deleted it. The most recent version can be found here:
http://www.tomtom.com/services/service.php?id=16&tab=87
Once you have Home installed, reconnect your unit and turn it on. It should ask about connecting to the computer. If it does NOT ask, use the reset button on the bottom of the unit and try again.
IF the device is recognized by Windows, let Home run. It will notice that you do not have any firmware on your unit. Go ahead and allow Home to install firmware only. It MAY not bring you up to the current level on the first pass. If not, disconnect and reconnect until you are offered 9.430 and it is installed.
Once you have all of that accomplished, you will be ready to restore your map from your PC's hard drive, and your 720 should be functional again. Let us know how you do with the previous steps first.