Just a thought, does your charger have the option of 6V and 12V output? 6V won't charge a 12V battery. Unfortunately I speak from experience...
If the battery is just dead, it should recharge. On my first car I left my headlights on when I was out on a Tuesday, and for some reason the car wouldn't start on Saturday
. I boosted it, and ran the car another 6 years on that battery, never changing it. But, that was a long time ago with less electronics. If the cells have started to short out then it is probably shot. Some people claim around here the life expectancy of batteries in seasonal vehicles (aka toys) is three years, I usually get about 4.