I had it happen 2 times but with only 1 speed.......try replacing the reostat /fuse thing on the fan.....about 10 bux easy fix behind the glove box....hooked to the fan wire junction........did it to my tj and liberty.
Could you give more description then this? Does the air blow but its cold? Is it warm but not hot? The air does not blow on any setting. It only works on high? Or some combination of the above?
You could have blockage but, without knowing more its hard to tell which way to go. My 98 had that problem of blowing barely hot air and after a few radiator drains it cleared up.