Most likely your battery has become discharged because your Converter (battery charger) has stopped charging it. This could be because the circuit breaker which provides 120v power to the converter has tripped, or because the Converter has died.
Another possibility is that the in-line fuse, normally found within a foot or two of the battery, has blown. It normally looks something like this:
and may be resettable (look for a button on it).
(If you don't have a voltmeter/multimeter yet, now is a good time to buy one.)