Heat is blowing out cold air

I have a 2003 Pontiac Grand Prix Gt. It has a 3.8 engine. I have notice that when I trun on my heat the ac button is always on and I can not turn it off. The heat used to work even when that button was but now all that is coming out is cold air. What could be the cause and how do I fix it?

Thank You


ps: it has 89,000 miles on it.