Had my Prime about 18 months but haven't used it since new. Tried to charge it last night and.... Nothing. A visit here indicates I'm not alone. So this morning I tried an old trick and it seems to have worked. I put the headset under my armpit for about half an hour to warm it up, put it in the charger and hey presto, I've got a red charging light!
I'm confident it will be fine but if not, I'll update later.
Hope this is helpful and sorry if you've already disposed of your expensive piece for kit.