Sounds like your present battery has bit the dust. SLA's do not take kindly to being deep discharged or left uncharged for long periods and it is doubtful it will recover. You can use an automobile charger IF (and this is a big IF) you can limit the current, in your case to about 2 or 3 amperes else you will fry it. Check out www.rcbatteryclinic.com for some good info on SLA as well as other types of batteries. If you get a new one, remember, do not deep discharge it, as soon as the open circuit (no load) voltage falls to 12 volts, recharge.