Have you called Garmin support to see what they'd offer you to resolve your issue?
sometime normal methods did not work, so its better to ask them .
otherwise as said above please repeat the steps.
1. Power OFF your Nuvi.
2. Press and keep holding the most right upper corner of the LCD screen
3. Power ON your Nuvi but DO NOT release your finger either from Power button nor from LCD screen (keep both pressed until the procedure ends)
4. You will notice Nuvi boots in Pre-Boot mode. Now quickly connect your USB cable. Prior to that make sure you have the latest Garmin USB drivers installed on your PC