RE: Help, reversed charger polarity
Inspect the pack for overheating damage. If it looks good, place it on a low tech C/10 slow charger for 20 hours (do NOT use a fast/peak charger). Then properly cycle it and recharge again. If the offending charge rate was low, the pack should come back to life (may take a couple of cycles).
You may have gotten lucky and blown the internal fuse. If that is the case, install a new 3A 5mm fast blow and you are good to go.