Garmin GPS model Nuvi 2757LM won't update.

Garmin GPS model Nuvi 2757LM won't update.

Could not get computer icon on Garmin, for map update. Garmin told me to leave it plug into the computer for 20 hours. It didn't help. I'm thinking battery. Back has ten, six pointed, torque head screws. I never update the maps before. It was working fine five or six month ago, returned from trip and placed unit in house.