[Jimmy Cao]

There's 2 possible reasons why it's dead.

1) The terminals are loose or there is some disconnection in the terminals/leads. This is easy to fix. Just tighten whatever is lose, or replace the wires if they're broken.

2) The battery is internally disconnected, or completely shorted out. Lead Acid can build up on the sides of the battery, and if they fall off, they can completely short out the cell, resulting in insane loss of voltage.

Usually, however, if you battery is only internally shorted, it would still have a very low voltage (not 0), so I do not suspect this. Check the electrical connections, and if they're good, I would suspect that there is a disconnect somewhere inside the battery, and such an issue is hard to fix.