A battery that has been badly discharged may not come back. Have you left the Tender on the car for 24 hours? It should be a low amp charger (about 1-1.5a) which, if the battery will take a charge, should get it up to 100% in a 12-24 hours. I have a Batter Tender brand trickle that I use on a motorcycle in the winter, and have also used it on cars that have sat (or had interior lights left on for a few days which nukes the battery nicely
Agreed on a deep discharge jump: typically can't just let it idle (unless it happens to have a really strong alternator). Load the car a bit to provide more jumping juice and see if that helps.
- b