Is it seen in preboot mode? To check:
  • Ensure it's fully OFF, not just in standby (hold power button down for up to 10 seconds to force it off it necessary).
  • Open Device Manager in your PC.
  • Plug in a micro-USB cable to a REAR USB2 port (don't use a front port, or a USB3 port or a hub - connect directly to USB2 rear port ONLY).
  • Hold nuvi screen's upper left corner and connect cable to nuvi which will power it on in preboot.
  • Watch Device Manager to see if "GARMIN Device" appears in the list and also note if there's a Device Connection sound.

It may only stay visible in Device Manager for a short time, some device series it's very brief but most are between 10 and 30 seconds.

If you haven't got preboot nor Mass Storage Mode it may indicate a USB hardware failure.