You are saying you connected your Zumo 550 to computer via USB cable and then deleted all files read as an external drive? Or instead moved those files to your computer C-drive? If you are sure no files now exist on the Zumo, it seems worth trying reinstalling to restore function before returning to Garmin for flat-fee repair. Files names from a known functioning 550 unit (I have 2) could be copied onto a Word doc as a checklist to ensure you still have all essential files, i.e., not including custom POIs, MP3s, pictures etc. Its unlikely both of mine match each other exactly, so yours would be somewhat different too.
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2004 R1150RT - Biarritz Blue