So I had Best Buy install the AVIC 7200nex at the end of July.  Everything was great until a few weeks ago  I can no longer get AM stations.  The HD stations rarely connect and the FM stations usually become static.  Why did this all of a sudden happen??????  

Why?  I don't know.

Check to make sure your antenna is connected, i.e. fully inserted, snug fit, etc.  Wiggle, rotate, or otherwise cause contact points to rub.  Check reception while doing this by keeping tuned to a weak station that you know reception should be better.

Another possibility is your vehicle may have an amplified antenna, and the amplifier lost power (blown fuse, open short), or perhaps went bad altogether.

If still no luck and you have the means, temporarily connect a different antenna to the head unit.  Or connect your vehicle antenna to a different head unit (a bit more involved).

Of course, there's always the option of going back to Best Buy and have them do the troubleshooting.  Not sure how long they warrant their installations.