Heater not working on 2000 Silverado

I tried turning on the heater in my 2000 Silverado 1500 (5.3) and it just blew out cold air. The temp gauge was running normal (about 190) and the antifreeze is full. With winter right around the corner, I want to try fixing it soon. Not sure where to start though. Any ideas? Thanks

id look for a broken vacuum line going through the firewall first. can you hear the blend door moving. if so you might want to flush your heater core.. specially if it still have dexcool in it. orange coolant..

I back-flushed the heater core and it fixed the problem, easy and cheap fix. Thanks for the help