You may have run out of gas. Go to your meter and check how much credit you have. If the meter shows a negative amount you will need to top up your gas card. Take your gas card to your local top up point and add credit to your gas card.
Watch this helpful video which explains gas meters in more depth including how to top up your gas card.
2. Check ball valve lever is in the on position
The ball valve is a lever that switches the whole gas supply on and off to the house. Watch this video to help you recognise the lever and then move it's position to see if it switches it back on.
3. Call national grid
Call National Grid on 0800 111999 to see if there is any issue with the gas supply.