I say thermostat issues because there can be several.
The first is simply that the thermostat has stopped working.
If there’s a short in the electrical line or the batteries are dead, then the thermostat won’t turn on and can’t send a reading to the furnace. Without the thermostat, the furnace won’t ignite.
It could also be that the thermostat needs calibrated.
If you set it to 74, but the unit thinks 74 is 70, then it won’t kick on when the temperature hits 73. This is a mechanical malfunction and just needs to be reset.
But there’s another possible problem as well.
It might be that the thermostat was installed improperly.
Thermostat location is important to effective use. If it was placed in the sun, the thermostat will think the house is warmer than it is, thereby causing the house to be cold.
And if the thermostat is placed near a door and a gust of cold air washed over it every time someone goes in and out, it’ll cause your furnace it kick on prematurely.
The thermostat is a small part of the furnace, but a very crucial one, and can often be the source of furnace problems.