Have a Honeywell thermostat TH6000 series. Has been working for about a year. When the thermostat clicks, Heat on, the furnace doesn't go on. Have an oil furnace. Had furnace serviced for the winter 2 weeks ago and worked fine until now. What could be the problem? Thanks for any input

There is no pilot light in an oil furnace.

First thing, do you have oil? Check the level.
Second, is the power to the boiler on? Check the switch(es).
Third, press the reset button. Does it start?