ac and heater dosnt work on 2001 GMC Sierra 1500

my 2001 gmc ac and heat does not work any ideas on wats wrong and how to fix?

Asked by for the 2001 GMC Sierra 1500
What do you mean by a/c and heat "does not work" on your 2001 GMC Sierra 1500? Is there no air flow? Is the air flow incorrect? Is the temperature incorrect?