First check the voltage against its terminals with the help of some multimeter / Voltmeter. It must be 12 / slightly above, when car is in OFF position. If its less than 12, then it might be possible that some of the cells in the battery have gone dead.
If Yes, then try disconnecting the cables and connect them with other battery. If car starts, then it means and confirms that something bad has happened with your car's battery, and it needs to be repaired / replaced.