blend door motor may be bad. is there any noise coming from behind the dash??
Usually if there is no heat and the temp gauge doesnt move it means no/low coolant.Check the coolant level again, it may have gotten low and now it has an air pocket in the engine. Do the heater hoses both get hot?

Roy


https://www.2carpros.com/articles/car-heater-not-working

Took it in yesterday and had the radiator flushed. When I left the heat was not working. The front and rear were blowing but it was ice cold and the middle was not blowing out any air. All three were working fine before I the flush. Called them to and told them what was going on. On the phone they explained to le about air pockets. Took it back to them and they then said sometimes when a radiator is flushed it will knock things loose and can clog the heater core. They said they went ahead and flushed the heater core so the heat in front was working. They said the rear was still cold because has more then one heater core. When I left the second time the heat in front was working and continued working for about 10 minutes before blowing ice cold air again. Took it back to them today and asked why they didn't tell me before flushing the radiator that it could cause issues and he just got a smart attitude about it. What did they do to my Navigator?

I feel it still has an air lock in the system, cut your losses with this so called mechanic and get one that can bleed the system of air for you.

What is the procedure to flush the heater core?

To really flush it clean you can remove the 2 heater core hoses where they enter and exit the engine. The you can find the inlet by looking for the heater core valve near the firewall. Set the heat all the way to the hottest setting. Then put a graden hose up to the inlet hose and let it flush until clean water comes out of the outlet hose.