Ok, I'm reaching out to all of you Moz'rs for some help with this one.

My client's site has dropped off the face of google in a real short period of time.  It went from page 1 (avg rank 3 to page 6 (avg rank 50) and below in the matter of 2 weeks.

Here's some facts:

1.  DA is a 22 and homepage PA is a 31.  It outranks all other sites in its competitive set.

2.  The homepage used to be the page that displays for keyword searches, now its the FAQ page, which has a lower PA of 23.

Why has the home page seemingly vaporized?  And, why is the FAQ showing as the first result?

What should I start checking.  I feel paralyzed, not sure where to start.

More info:

a.  There are no alerts present in Webmaster Tools.

b.  For some reason the homepage (domain.com) was 301'd to domain.com/home.html.  Domain.com is indexed by Google, however, domain.com/home.html is not.  If this is the issue, what is the best way to handle it?

Thanks in advance for your help!