You should need the backup of Non Vol memory coz firmware update or Mapupdater corrupted it
37xx in Nuvi infos means that your Nuvi lost Non Volatile memory. This Non Volatile memory includes model number, registration code and unlock codes for factory preinstalled map. Since there is no model number, Nuvi software doesn't unlock features which are model dependent. NV rgn#41 contains 3790 model number.
It can be restored in 2 ways, by sd if someone helps you, or by factory cable,but both ways are a bit complicated., better to send it to Garmin if warranty is still valid.

EDIT: have you checked in Nuvi infos if you really have 37xx instead of 3790?