Everything comes out the main front vents - Not heater or defrost vents (90-s13)

[Peabody]

Hi, I have searched this I have not found any results on the forum. I have been hearing a strange sound for the last few years when I would switch from lower vents and press the upper vent buttom like a cracking sound and then after a few seconds the air would change to the main vents from whatever place they were. Now everything comes out of the main vents and I cannot get the heat to come out of the bottom or air out of the defrost vents. I get no sound also I guses because the air vents are stuck on the main ones. I do not know anything about AC but can I check or clean something to get it back working? THANKS