It may be that your charger is not "seeing" the battery. Hook this "dead" battery to another charged battery in parallel. Negative to negative and positive to positive. Then put your charger on the two batteries and let them charger overnight. The next morning disconnect the two batteries and then run your charger on this "dead" battery and see if it will take a charge.
Most chargers have a minimum voltage that they must recognize on a battery before they will start charging.
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