Q: No heat after adding coolant and thermostat almost to H

Hello. If the coolant was that low then that will cause the heater to not work. The most important thing at this time would be to determine where the coolant went. For it to be this low that means it is either leaking onto the ground or the engine is burning it. I usually start with a visual inspection to see if I can find the leak. If nothing comes up then I do a pressure test on the cooling system as that is more likely to show the leak. If nothing is still found I do a dye test and then a block test. This will tell me if there is something going on inside of the engine that may be more serious. If you need to have this cooling system problem looked at, consider YourMechanic, as a certified mechanic can come to your home or office to diagnose and repair this cooling system problem.