Are you having problems with your furnace or heat pump?
You should probably call one of the heating and cooling companies in your area if your HVAC system has stopped working. The last thing you should do is to try and fix it yourself. First off if you are not certified to handle AC units and heat pumps you will probably just make matters worse instead of improving them. Also, you will most likely not even know what to look for to see what is really wrong with your heat pump or furnace.
Now, if you are dealing with a central air system you really need to make sure you contact the right people. If you or someone you hire messes up while trying to fix your air flow system you will be stuck with a huge financial bill and a lot of work to fix it properly.
This is why it is best to contact either a commercial or residential AC repair company from the very start. When you do contact them make sure you ask to see come credentials before hiring them for the job.