Good idea to change the pollen filter first...also look check to see if any plastic pipes have come off while you have the glove box out..
Can you hear the fan going when you move it between slow and fast in steps..
it could possibly be that some one has had the heater/air controls off and not put the right hand dial back in properly ...
and is the air recycling button(the one on the left with a car and MAX written on it )work properly(can you hear the flap close and open...(if this is stuck closed it wont let the air through Fast)