Garmin nuvi needs Garmin drivers.
First check if you switched your Nuvi to "Mass Storage" mode
[Only registered and activated users can see links. ]
Check in the device manager if there is any bang sign next to this device.
You may try using another usb port and check.
You can try using another USB cable and check.
You can also try running the 7 Hardware Troubleshooter to diagnose the issue better.
Code:
Please Login or Register to see the links
If some driver is missing you can run Windows update
But you should tell us what happened and why your pc doesnt recognises nuvi anymore.