First you must make cure with the right firmware according to your device HWID,or your old GCD shows.
If you dont know it use [Only registered and activated users can see links. ]
Then flash cure by Updater.exe in preboot, format nuvi following this: [Only registered and activated users can see links. ] -
After formatting ,reflash original firmware in preboot with Updater.exe :Nuvi cant boot with cure firmware, it's made to remove problematic files plugged USB to pc or to format nuvi.
Then run Webupdater.
All the steps are necessary in your case I think, but you havent written what happened before having this problem , so I cant help.
This in very few words, but all is explained better in post n1 (useful links in post n.3 too)