I already checked the fuse. It was OK, but the heat is still not working? what should I do next?

SWITCH MAY BE FAULTY OR WIRE OFF UNDER ONE OF THE SEATS??

Reverse flush the heater core (videos on YOUTUBE).
If that doesn't help you may need to replace the heater core (Major job)

if you already checked the fuses and found them to be good then you may now want to check the connectors under the seat. You will need a probe light to so this. The probe light will light up if you have power coming into the (+) terminal end. Watch this video to learn how to use a probe light tester.
https://www.youtube.com/watch?v=Y0HjaR0tj7Q

Check that the wiring harness plugs under the seats haven't become disconnected. Next thing to check is that the harnesses have power when the controls for heat/cool are activated. Also check the interior fuse compartment for blown fuses.

there is a wire that simply slides in on the back of the lighter housing that may have slipped off

You may have a problem will the heating grid in the seat.It may have to be replaced.Also you need to verify that your switches are ok and that all your connections are good.

might be low on freon that would cause it not to switch on have someone check it with guages

the flasher likes to fail. Check that next.

did you check the fuses? that would be the first thing to check.

Someone has disconnected the seat heating plug for a reason, probably for the same reason and situation you now find yourself in.

But don't panic, more than likely you have blown a main fuse in the relay panel so check that out before doing anything else. You are unlikely to have blown the wiring and there isn't much else under the seat to disturb.