EMERGENCY SEO

After revamping our site we have incuured a huge drop in the google serp for a very specific keyword, the site has been indexed, contains over 2500 pages, all with seo titles, meta descriptions, the site has perfect on page seo i.e a single h1, couple of h2, h3 etc all containing keywords, img alt tags, title tags, i has a tag sections, and seperate cat section, with unique permalinks and category base removed, text-to-html ratio is low however, it has a blog section and QaA section fot regular Fresh Content however the site is not ranking anywhere desired for the target keyword all the other seconfdary keywords are fine but the MAIN KEYWORD is no longer in top 300 resuls despite getting to page 5...i would need someone to have a look ab this remedy the problem immeadialy...301 redirection is in place, htaccess is working fine and xml sitemaps have been submitted to both GWT and BIng