Our central heat and air unit... works sort of. The air conditioning still comes on and the fan works but the heater won't come on. We hear it try to start but it won't kick in. What to do?

Brand and model number of furnace.
Better description of problem with unit. (just saying we hear it try to start could be many different things.) Be exact. Stand by furnace when activated and see what happens. Then report back to this post.