Batteries are not maintaining the charge or check the level of liquid inside your battery. Use hydrometer to test the specific gravity as it should not be less then 1250 and more then 1300.
1) If your car can start on Petrol and not on CNG then the problem is with your battery.
2) Once you switch on your car and your starter moter not showing any response then probably then battery does not have enough charge to produce a cranking Ampers.
3) clean your car batteries terminal.
4) Also drain your battery Acid and ask the local technician to refill it.