Front air vents and defrost for heat and AC not working

Problem began first with just the Max AC setting that stopped working. Then all the front vents stopped blowing out air for a few days, but then started working again. The problem has now returned and none of the front air vents, front floor vents or defrost will blow out any air regardless of which speed setting I put it on. When I put my hand over the vent and turn on the heat, I can feel a very faint amount of warm air, but it is not blowing the air out into the cabin of the vehicle. The rear AC/heat located behind the center console is blowing out air and works just fine. Any idea as to what is causing the problem?