leggeron, swathdiver is right.. try hitting it with high current ... Hook it up to a car battery charger on 2-6 amps 12volts..I have used up to 10 amp charging but only for 20-30 second intervals. Stay right next to the battery and as SOON as it starts to get warm take it off.. Keep an eye on the plugs too, they might get warm with the charging..

I've done this many times to revive batteries, it does work just don't leave it on and walk away.. You have to stay near it to monitor the temps.
Let it sit for a couple mins after it's done until it cools down then run it.. Don't run it right down to nothing.. After you've run it, let it cool down completely then charge again..