My heating does not come on - no communication between boiler and thermo?

My heating has not come on today. This has happened before about a year ago and I called the plumber who installed the boiler and it took him not even 5 mins to get it going again after putting some sequence in the thermostat.
There is not communication between the thermostat and the boiler. However, I do have hot water. Need advise on getting it going again?
Thanks