It's possible that your battery is too far gone to be recharged and you'll just have to buy a new one. Then, going forward, you should buy a trickle charger (i.e., Battery Tender Jr) that you just keep hooked up 24/7 to the battery during any lengthy times when you're not riding. Don't just put a charger on "every now and then." Batteries live longer if their charge is kept relatively constant, rather than them being run way down and then fully charged back and then run way down again, etc. etc. A Battery Tender Jr runs somewhere around $25... lots cheaper than a new battery every year or so.