Take it out, place it on anything but cement and put a 1 or 2 amp trickle charger on it. Preferably one that "monitors" the battery and will shut off when it is fully charged.
When a battery sits without being charged the positive and negative plates will become encrusted with sulfation.

Lead sulfation occurs when lead sulfate cannot be converted back to charged material and is created when discharged batteries stand for a long time. When the state-of-charge drops below 80%, the plates become coated with a hard and dense layer of lead sulfate that fills up the pores. The positive plates will be light brown and the negative plates will be dull, off white. Over time, the battery losses capacity and cannot be recharged.

Another thing that happens is..if the sulfation builds up on the top edge of the plates it can join with the sulfation from the plate beside it. the fact that one plate is positive and the one beside it is negative is a bad thing. This shorts out those plates and will cause a bad cell.
Bike batteries are fragile enough in good condition..it doesn't take much to ruin one.