Sorry about your Charizard, It could be swtuck in there for a LONG time.
First of all, changing the battery does not reset the memory.
First, If you press certain buttons all at once on the Pokewalker menu (I THINK it was li9ke B, R &L and Up or somthing), It will force you Charizard back to the Pokewalker without connecting though.
The watts and levels you gaINED while Charizard was in the Pokewalker will not be recorded, :(, but your Charizard will be safley returned to the place in the PC where yo left it.
If you do this and try to forget about the whole incedent, I may seem like your Charizard never even went to your Pokewalker.
(If youre worried about the Pokewalker nor responding to the DS in future connects, It will respond, luckily, so DONT stress yourself).