1996 Buick Park Avenue Heater blows out cold air

Few days ago, I got in the car and turned the heater on and notcied that the heat was not working. The car is runing like it should as in the temp on the gauge. The driver side is blowing out lukewarm air and the passanger side is blowing out really cold air. It just happened all of a sudden, I have been told by some people that is could be gear that closes the air off for the change in the air into the car. I just wanted to know what you thought it was and if it is something that can be replaced myself or something I should take to a mechanic. Thank you for your help.