What can cause my heat not to work. There's no coolant leaking anywhere r cracks

What is your question?

Wednesday morning of this week on the 21st of 2016 I was getting ready for work which was around 4 in that morning. Sort of cool outside so I crunk up my car, turned the heater on max along with the defrost. At least bout 15 mins my car was running. Blowing out cool air instead of heat. Mu coolant level is full and there's no leaks anywhere.