You have probably been banned for some arbitrary reason. If in Yahoo results there is no link to "cached" page, the page is not actually indexed by Yahoo and therefore no one can find your page in a search for key words in the page text. Yahoo also indexes just the URLs of many sites that they don't index, presumably to be able to claim the "largest" search index.
If site:yoursite.com returns no result and your site isn't quite new, then most probably it has been banned by Yahoo.
Your best bet is to probe into the factors (eg. Duplicate content, linking to bad neighborhood, massive keyword stuffing, hidden links to name a few.) that caused your site get banned and clean it up. Then send a re-inclusion request at email@example.com and wait until it re-indexes your site.