That's either your battery or your alternator.
Your alternator charges the battery when the engine is running. If your alternator is goosed then the car will drain what power is left out of the battery, flattening it.
Or
Your battery is dead (different from flat). Your battery turns the starter motor. If the battery is dead, it hasn't got the power to do it. You can't charge a dead battery.
So you need to find out which one is causing the problem.
A multimeter will do that for you.