2005 Chevy Monte Carlo No Heat

It sounds like your heat is working just fine; your problem is your level of coolant. You have a coolant leak somewhere.
Try to check for leaks when your car is running.
Make sure there's no leak at the water pump, somewhere on the radiator, hoses, etc.
Have someone pressure test your cooling system because you could also have an internal leak (crack cylinder head, leaky head gasket).
Also, have your radiator cap tested as well.