When you say the car won't start, what happens when you turn the key to the START position?
Do you hear a click under the hood?
Do you hear the starter motor sort of "grunt" as if it'* trying to turn the engine, but doesn't have enough "oomph" to do so?
Do the car lights get very dim when you try the starter, or do they not change in brightness at all?
If they get dim, get someone to put a digital voltmeter right on the battery terminals, and get a reading with the key off, and a second reading while you are holding the key in the START position. Then report those readings.