>>28696716 (OP)
You might need a new battery. Lead-acid batteries tend to lose capacity permanently if left uncharged for long.
If it's been empty for a month, I'm not sure if I would trust that thing to hold charge any more.
If you don't have one, go buy some cheap "smart" charger and see if it can charge it up at all. Doesn't need to be fancy, some $30 unit will probably do if you're not in a hurry.
If the battery even accepts charge, let it charge to full, disconnect the charger and check again the next day.
If the battery's voltage drops below spec quickly after being disconnected from the charger, you can go buy a new battery too. You can then use your new charger to maintain it every now and then so this won't happen again.