Is it on the same domain name (URL) as before Mav? Google has a nasty habit of "sandboxing" new sites to see what happens with them before they are included in the indexes. That can be up to 9 monoths, and there is nothing you can do about it.
Another thing is that they may have got a new algorythm (happens from time to time) and are going though reindexing, in which case it will appear back shortly.
There are help pages on Google for webmasters that are quite informative.
All this assumes you have not been tempted to use tricks to try and fool Google. If you were up in the top few results before then I am guessing you are doing it honestly. Devices such as tiny background colour text and hidden content are outlawed and will get you blacklisted.
Threaded Mode