It looks like something quite bad happened to Google search on June 27. There are long threads about this event on Webmaster World.com: June 27 - changes and June 27 - changes Part 2 but everybody can only guess why certain sites have been hit and certain have not.
Unfortunately Multiplex Systems Store is one of the sites that suffered badly from this event. It produced top 5 search results for a wide range of keywords and products for several month now. However, after June 27, almost all results went to nowhere, almost zeroing traffic from Google. The resolution is yet to be found.