Unfortunately, I have some bad news for you. You probably have a blown head gasket. The outflow from the cylinder that has the problem stops all water circulation. See if the circulation starts again if the radiator cap is removed. This will vent combustion gases and allow normal flow. So if you run the engine with the cap off, and heater function returns, and then you put the cap back on and the heater function quits, you almost certainly have a blown head gasket.
To confirm this, call around to some of your local garages. Tell them you suspect a head gasket leak and want to know if they have a chemical tester. If they do, take the car down to them and to have them run the test (shouldn't cost too much, and it only takes 5 minutes.) They place a glass tube with a rubber seal over your radiator tank opening or your expansion tank opening, put in some test fluid, and use a rubber bulb to suck air through the test fluid. If the fluid changes color, it means
there ARE combustion gases in your cooling water, which indicates a blown head gasket. What do you do then? Well, the first step would be to try one of the head-gasket-sealers-in-a-can that the auto parts stores sell. This is the cheapest for a first attempt. Will probably run you $10 to $70 depending on which magic sealer you use. If this works, you are good to go. If it doesn't, the next step will be a head gasket change.
35 years as an airplane mechanic and also messing around with cars all that time.