These pages explain two heating problems that are easy to mix up. They are not a list of every repair.
Boiler Running, Radiators Cold
The burner is on, or the boiler is hot, and the radiators or baseboard stay cold. Steam and hot water fail in different places, and a few cold rooms are not the same as a whole building with no heat.
Read why a running boiler can leave the radiators cold. Repair for that equipment is steam and hot water boiler repair.
Furnace Starts, Then Shuts Off
The burners light and then stop. Shutting off after a few seconds is a different fault from shutting off after several minutes while the blower keeps running.
Read why a furnace starts and then shuts off. Repair for that equipment is gas furnace repair.