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If the drop was late January to early February most likely related to Google's PBN update (blog network). A lot of sites have been hit, some cases badly. It is most likely stopped counting the blog network links that were keeping the site at its original position, Google is probably now ignoring or even penalising some or all of these links. The caveat is, due to the nature of the update it is unlikely that disavowing the links will help, probably the best option is to build some new and better ones.