What is the operational relationship between boiler and thermostat?

I have a Worcester 28CDi combi boiler with a Honeywell T40 thermostat. I wanted to change the thermostat to a digistat + for more accurate setting. I was sure that I had connected the digistat correctly but the boiler started flashing the ingintion fault light. I then reinstalled the old thermostat exactly how it was before but the heating was not starting. The boiler has no problem heating the water but the ignitfault light still flashing and no heating.

Can you help?