IIS on 2003 SBS server returns "Service Unavailable"

I have an issue with my SBS server.  I have been working on it, trying to utilize OWA, etc...  I finally got everything working, told the users that OWA is up and ready to use, and then a few weeks after I get a report that its not working.  I checked, and it is infact NOT WORKING.  I get a "Service Unavailable" error.  There are no indications as to why.  I can't get WWW services to come up on this server.  The IP addresses are all the same.  Please help me troubleshoot this.