2004 impala heater and AV not blowing air

I have a 2004 impala. For the past month the heater
would stop blowing air all the sudden (while
driving). I read up a little bit online and saw some
other people had the same problem and if the
banged on the underside on the dash board it
would come back on. This usually would work for
me in the morning after the cat had sat overnight.
The blower would start right back up. But the other
day it stopped and I have not been able to get it
start blowing out heat again. It doesn't blow out
cold air either. Which part does it sound like needs
to be replaced? Thanks!
Andrew