hober

I have a mz-nf810 about 4 years old. It hadn't been used lately. I tried turning it on and it didn't work so I figured battery was drained. I plugged it in and battery wouldn't start to charge. (It plays fine being plugged into wall.) So I thought the battery was bad. Got a new battery and it won't start charging either. I borrowed a friend's battery which was definitely working in her minidisc - but mine still wouldn't play. ANy ideas on what to do now? Thanks.