Heating on max, all the dials turned up and nothing coming out of any vents? Or air coming out of all the vents except the front screen or side clearing vents? I usually just hit the max demist button when mine is steaming up but it could be a few things. Could be a broken flap for directing the air if it is just not coming out of a certain vent, could be a fuse if there is no air coming out at all or maybe a belt that drives the air conditioning. All things you should get BMW to look at as I doubt there are easy DIY fixes other than checking your fuses if there is no air at all.
