It sounds like it was stolen. I guess nothing you can do about that...
As far as the key goes, open the battery up and see how it connects. Likely it's a very simple mechanism and you can take the lock out and rewire it to operate without a key. I'd do that first and see if the battery is any use. With used batteries it's a total crap shoot whether they are worth hanging onto. Then take it from there. Don't put money into it unless you know it's worth salvaging.