This is one of the most frustrating things that can happen to your site. Believe me. You will surely test the limits of your patience and your skills at site-cleaning and getting in contact with someone who can help.
In the company I do web work for, we have a (current) problem with Yahoo. All of our domains (about 10 niche sites in the same industry) were dropped from Yahoo on Jan. 1 of this year. The big problems are (a) we are all over the other engines in excellent position, (b) we used to be all over Yahoo in a similar fashion, for several years with these domains, and (c) the index pages from each domain are still in the index, but that's it. Titles and descriptions for the index pages are showing in a
search in Yahoo, so officially we aren't considered to have been "dropped" ... but in effect, we have been. This greatly limits the remedies we have available to us, as it hasn't been "sandboxed" (a term from Google users where the file is in the index but only the domain address shows up, no title or description), and so it is responsive to the Yahoo tool that determines if a site is in the index or not. Yahoo's tool says our domains are
in the index.
This is 8 months of trying to get in touch with YST (Yahoo Search Technology is the "algorithm"/natural arm of the company, and YSM, Yahoo Search Marketing is the PPC/paid arm of the company) and failing to even get an email address that goes to a human. I speak monthly with YSM reps, as we spend a ton of money on Yahoo PPC, but even they can't get anything over to the YST side.
Anyway (whew), my point is that even if you follow all of the rules laid out in the article and in the various guidelines, there are still instances where your site will disappear from the index, and you will be pulling your hair out trying to figure it out. Was a complaint filed? Is there a bug in the system? Did a flag get placed in our database entries? An "oops" moment?
Very frustrating. On the bright side, Yahoo has brought us almost no business over the years from a "natural" search standpoint, so the impact on our business is very small. Still, it really sticks in my craw.