Can someone tell me what is wrong with the vents in my 08 Jeep Liberty?

Heat and A/C work just fine blowing through the defrost vents and the floor vents but when the knob is switched over to the front vents no air blows out at all. You can hear it inside like it is working but nothing. I was just wondering if anyone mechanically inclined could tell me what the problem could be and if it is going to be super expensive to fix? I will be calling the mechanic to get it in the shop on Tuesday but I want to brace myself if it could be an expensive fix. Thanks!