I have a lennox heater I have to turn it off and turn it back on to get it to heat. What is wrong and how do I fix it.

Need to know model # it could be that you sensor is dirty

A great place to start is the Flame Sensor,
Disconnect the wire(s)
Remove one small screw
Clean with steel brush, sand paper can cuase crystalization and force the component to go bad sooner
Put back in place and see what happens