Leaving the battery
hooked up without some sort of charger will NOT "boil off" the battery.
Simply charge the battery with a battery charger. Chances it will ok, it's just discharged, which might shorted the life some. A good battery charger will also do minimal battery fault testing.
You can also pop the cover that allow inspection of the liquid levels, refill with distilled water and again charge with a good battery charger.