1. Check room stat is not turned down and also check the timer is turned on.
2. The pilot light may have gone out, check and to re light, depress the black or grey button for 30 seconds and release.
3. Then you should check the pressure gauge on the boiler. The pressure should be between 1-2 bar. If it is below 0.5 bar, the boiler will lock out for safety. You will have to refill the heating system to 1.5 bar by using the filling loop, which is generally under the boiler, maybe in a cupboard underneath or beside the boiler. The filling loop will be a flexible hose with either two taps at each end or a lever. Once the correct pressure is reached, you must turn it off fully.
4. If you have air in the top of a radiator, you can vent it but you must the re pressurise the boiler to 1.5 bar as explained above.
5. The boiler may need re setting, reset button different on each boiler, refer to manual.