No Heat in my 2003 Chevy Silverado 2500HD

I have a 2003 Chevy Silverado 2500HD with a 6.0 engine. This morning I Started my truck and let it set for 15 minutes to warm up and defrost the windshield. When I went back out there was no heat at all coming from the defrost or the heater. I drove it around after scraping the windshield and still no heat. It was working fine before, what could it be?

Response:

As long as your coolant level is full, you may have a bad thermostat or a clogged heater core. With the engine running at normal operating temperature, check the heater hoses to see if both sides are hot or if one side is cold. If one side is cold, then you have a clogged heater core and it will need to be replaced. Look for about 8 hours for this job.