Need urgent help with GWT!!

Hello yesterday my site really dropped out of top 3 positions on google for most of my keywords. Im confused on how 3 of the 6 stayed and the other 3 that were also locked in first position dropped? I got this error in GWT Increase in not found errors , basically stating about 404 errors returning not good. Someone has been negative seoing my site hardcore and i think what happened was the sites that had the negative seo on them some of them may have overloaded and crashed causing those broken links and 404s. If anyone knows how to fix this or can help it would be greatly appreciated! It shows I have 66 crawl errors in the past few days all from this guy please help! Thanks!

Do an audit of your backlinks, comiple a text file of all bad links/irrelevant links and use google disavow tool. this will help you receover ur site.