Yukon Floor Mats Why Won't Air Won't Come Out My Car's Front Air Vent's? ?

Why won't air won't come out my car's front air vent's? ? - yukon floor mats

I have a 00 'Yukon, put the A / C on full blast in front of the air, but only through the windshield, floor and roof vents and rear, but not. Can someone help me.