netmeg - 4:01 pm on Mar 6, 2013 (gmt 0)
Do you have an idea yet who is replacing you? Who is getting your rankings or your traffic? I'd take a look at them. I'd check to see which keywords/phrases are no longer bringing traffic (to the extent that I can tell) and whether the decline is sitewide, or only for some specific pages. Do you maintain a GWT account? Take a look at your search queries and your back links.
Are you in a competitive niche? I'm sure you do have a good site, but if the competition has heated up, having a good site may not be enough.