Vaillant ecoTec 831 I have hot water but no heating?

Please help! Our heating has suddenly stopped working. The display shows that the internal pump is running, the heating mode is active and the warmstart system is active.

No flame is showing (not sure if it is supposed to).

The radiator temperature controls are set to the correct setting.

My husband is an electrician and has just changed the thermostat so we know that is working ok.

Any ideas?