Why my sudden drop in rankings?

Trying to figure out why the heck my Google traffic on one of my sites just suddenly CUT OFF! Have a look…

I've gone into webmaster tools and see 'No new messages or recent critical issues.' for the site in question..So, I can't figure it out? Why would my site drop all of sudden when Google is saying nothing's wrong? Are there any tools I can use to further analyze?
Thanks!

See original post by FPForum