Hi, leisure batteries dont like going totally flat, all you can do is charge it with a proper leisure battery charger and see how it performs it may recover or may not it's really a case of try and see.
------------- Doing as little as possible for as much as possible...
Or leave it fully charged for a few weeks, to see if it has held the charge. There is probably about a 50/50 chance either way. I completely flattened mine twice with faulty wiring on my fridge. I thought it was the battery faulty and bought another one, until I discovered the faulty wiring. Recharged the battery and it's still going fine. At least I now have a spare, although the £50 would have been more welcome.