Happened to me alot of times on a rented car
Car starts on Petrol, I rev it a bit to convert from Petrol>CNG. Once I rev it, the CNG light switch just turns off, and the car starts using Petrol. I figured out to press the wires connecting to the little CNG switch, as Brother usmanuetian and wasilrkhan mentioned^.
I learned the hard way, by driving on Petrol, and once the petrol finished, I clocked on -.-
Good luck :)