I agree. I can't help feeling that that battery had some serious problems before you put it into storage.
Sealed lead acid do in fact store better than flooded ones, but a decent working flooded cell should still survive many months if fully charged before leaving. If it is in a poor state with high self discharge then it may expire unless more or less constantly being charged, it was probably near the end of it's life.
Much depends on what has happened, if all cells are flat then it will be badly sulphated and probably won't recover. More likely I suspect one cell is faulty and near open circuit but unless you have external link bars you can't work out where it is wrong.
If one cell is duff it may respond to charging and come back to limited life but it will never be something to rely on.
Flux