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I think you may find that connecting the Zumo device to a computer that has an internet connection active and the Garmin Express program running on it, may allow you to cure the problem.
At any rate, it is the thing to try under the circumstances, when you have this sort of problem, before falling back on sending it to Garmin Repair.
When dealing with an update for your Garmin device, it is best to simply download, install, and use Garmin Express.
and (if you have a Windows PC) click on the big blue Download button at center bottom of the page. (If you have an Apple, click on the nearby Download for Mac link.) And download Garmin Express' installer to your computer, save it, then double-click upon it to install Garmin Express.
Then start the program, Garmin Express. Then connect your Garmin device. Then use Garmin Express to update your device.
Tell me how that turns out, please. It may get it working, it may not, depending upon how severe the damage to the device is, etc. If it doesn't work, I'll have more advice to offer.