I have a 495, and had not used it for a month or two and when attempting to swich it on "battery not inserted" showd up as you mention!
I did the usual and took out battery tried to charge inserted not inserted battery! no luck, I was busy at the tme so had no time to send it back, when I tried it a week later before sending it back it was fine and has worked perfectly since, six months or so..
I would ring or e-mail the Garmin tech. support they are very good.