The gas is back on
Great! Now you will have to reset your boiler to get it working again. Go to your boiler and turn the on/off switch on the front of the boiler to the reset or ( R ) position. Hold it in the reset position for 20 seconds. It should then result to the on position.
If you have a Baxi Duo-Tec 2 or Potterton Assure, you will have a reset button, which you need to press for 1-3 seconds, which should reset the boiler.
You might need to do this several times before it will light and stay lit but try it a few times before considering reporting a repair.