How do I fix my heating system if one room gets very hot and others remain really cold?

The answer is in ScottE's brief note. Your house's problem is in the ducting and dampers. I suspect a few breaks (disconnected branch ducts).

Start at the heater and trace the duct runs, test each damper to make sure they are open, look for "blowholes" or disconnected ducts. Connect the disconnected, cover the holes (sheet metal patch pieces, screws and seal edges with duct tape), open the dampers.

If you do all that and still have trouble, call in a Heating & Ventilating contractor. Tell him to give you a quote for duct cleaning. (They have brushes on a wound cable something like a dry Roto-Rooter.)

What I suspect, at that point, is that something large has gotten inside a duct (someone dropped clothes down a floor register or an animal crawled in there). The HV contractor will find it and you'll have cleaner air circulating in the house.

The typical house should have the ducts cleaned every 5 years for the sake of Indoor Air Quality.

Check your furnace pipes. There should be dampers that you can adjust. The problem may be figuring out which pipe flows into which room. Get someone to help you. Ask them to shout down the pipe in each room & try to figure out which pipe the sound is coming from.

Then do the damper adjusting thing. See if that adjustment has the result you want, or re-adjust.

It may take a while to do this checking & adjusting, but it may be the answer.

Good luck.

first make sure your furnace filter is clean. next, adjust the floor registar openings. adjust the warm room registar to half open and see what happens. also do the same in the room that has the thermostat. it will take twice as long to heat that room allowing the rest of the home to warm up

In the cold room see if there is any air flow from the register. If not, the supply line has pulled loose or is blocked.

some form of fan feels like the obtrusive answer. i think of a ceiling fan may be the 1st decision yet even a pedestal fastened oscillating floor fan could probable help. G. Borders abode restoration, progression, & Handyman provider Plano abode restoration

There may be dampers or a broken connection at the trunk line.