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Hi Naomi  - You will probably get some various thoughts on this, but it's probably best to forget about the messages Google is telling you. Focus your time on earning quality links from topical sites. This will easily replace the value lost from your previous linking efforts. Without knowing all of the activities you've done for the site, it's hard to provide more reasons as to what could have caused the drop in rankings. Be sure to do a thorough audit of the site from a technical perspective and take a deep look at your backlinks to see what the issues might be.