I had good AC all summer long; now that it's cooler I need my heater to work.

Is the engine running at operating temperature? What does the temperature gauge showes?

Common problem that the heater core gets clogged. We have had some sucess with backflushing the heater core versus replaceing it.

CK COOLANT LEVEL THEN MAKE SURE BOTH HEATER HOSES ARE HOT AT FIREWALL TO START WITH