Battery tenders won't usually charge up a very depleated battery. A lot of "smart chargers" won't.
A guy can usually bring a fully depleated battery up with an old-school analog battery charger on it's lowest setting .3A or 1A. Let it charge like that on it's lowest setting for a half an hour or hour and then bump it up higher in a gradual manor. Once you've put some charge in it, your Tender should recognize it and take it from there. Expect it to charge a long time ...like a day, if it's that depeated. Don't charge over 10a
If it doesn't come up after that, it's probably dead. A lot of guys have revived a lead-acid patient doing as above.
For years our service mgr at work would replace 'dead' batteries from service vans. They'd sometimes go dead over the weekend from GPS units, etc. I'd take them home, bring them up (as outlined above), and they'd work perfect for many many years. We had a walmart next door and it was easier and faster to just send someone over & buy and install a new battery.