Heater problem on Chevy Venture?

About a year ago my van started blowing out cold air no matter how long it warmed up. Now it doesnt blow out anything. While in drive it will blow just enough air to defrost my front window after I have been driving awhile. My back seat heater blows good but the front doesnt at all. What could this be? If you think you know the problem could you give me an estimate of what it may cost? I want to get this fixed ! Very difficult to drive when your window is frosted up and you have to drive to get it to thaw! Please help!!!