The heating part of my ruud unit is not coming on. The pilot is lit and the cooling unit comes on just fine. Also the fan will blow whe we turn it off auto, just doesn't come on when we turn the heat on auto. What can I do?

My ruud unit is cooling but not heating. The pilot is lit but the heats won't come on. The fan blows when I turn it on but when it's on auto but not when it's. On auto. What can I do?