More than likely you have an issue with the thermostat not opening. I would suggest changing the coolant and the thermostat at the same time.
https://www.2carpros.com/articles/coolant-flush-and-refill-all-cars
https://www.2carpros.com/articles/replace-thermostat
If you change this out and the engine heats up, but the heat does not return, then you have an issue with the heater core most likely.
Check the temperature of the two hoses going to and from the heater core and they should be the same temperature. If they are not, then the heater core is plugged.
I am attaching the info below on how to replace the thermostat.
Please run through this info and let us know what you find. Thanks
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Tuesday, November 2nd, 2021 AT 6:26 PM