what do you have in Nuvi/About as model number?
Nuvi 40/50 and nuvi 42/52 load a different HWID firmware.
[Only registered and activated users can see links. ] --> HWID 1350
[Only registered and activated users can see links. ] --> HWID 1613
What was your nuvi original HWID and what is now?
You can try to flash the correct .rgn by Updater.exe in preboot, just drag the gcd in RGNTool and click on RGN button (upper left corner of the program) save it and flash.
Flashing a wrong HWID usually hard bricks a nuvi, but if it still works, you can try.
I guess you don't have NV backup, so IMHO if you lost model number in NV, it is almost impossible to restore it, unless you find a smart guy with your same nuvi model a service cable and some skills enough to prepare you the files to send you to flash them by sd.