My battery is dead; how can I bring it back to life?
0. Charge the battery after disconnecting and removing it. You may have a current leak on the bike somewhere.
1. Check that the leads are on correctly. (No, you didn't hook it up wrong... but you'd be surprised.)
2. If the charger has a voltage switch, set it to 12V.
3. Try a different charger if you have access to one.
4. If the battery has removable fill caps, add distilled water to the fill line. Only ever add water to batteries. If you add more acid, you upset the balance and will kill your battery even more quickly.
5. If the battery has removable fill-caps and adding water does not improve its ability to charge and hold a charge, buy an electrolyte checker. Follow the instructions. This device checks the acid content (and hence the ability to take a charge) of the water in the individual cells.
6. If the battery does not have removable fill-caps, you're done. Get a new one.
In general, check the voltage after you unplug and disconnect the charger. Shouldn't make a difference, but you want to eliminate all but one variable in testing.