My thermostat calls for heat, but the blower just comes on and blows air. No heat until I go down and manually restart the furnace. Any suggestions?

Oil or gas or electric furnace
What are you doing to reset furnace?

My thermostat calls for heat, but the blower just comes on and blows air. No heat until I go down and manually restart the furnace. Any suggestions?
It is an gas furnace. To reset it, I either turn down the thermostat and turn it back up again or I flip the switch on the actual furnace off and on. Any ideas?

If your unit has a flamw sensor try to clean it.

It has worked ever since I cleaned the flame sensor. Thanks