If your keyword is slipped away form 1st page to 2nd then no worries. Google have 42 data centers all over the worlds, the Updates takes place there affect a little bit for a short time. If your keywords fell badly like not in 100 then it is a matter of worry, you should check you last work done. Something went wrong.
Check your backlinks again, try to build links with Google Link Diversity Process. Add fresh content on the landing URL's, if possible revise the meta tags. Be patient and soon you will see the results.
If you are lost your ranking on most keywords, there so many reasons like lack of on-page optimization, bad backlinking etc.
so now you can try change your website meta tags and create backlinks on high pr sites which help you increase your keywords rank.
Did you change the title or keyword of any of your site page and then performed off page technique? if yes ,then this may be the reason of drop in ranking. because the same thing happened with my friend two months back.In that case you should wait for one or two weeks to take an effect.
Google always changed its algorithm so if you not follow and not update your website structure like meta title, description, content and other requirements then will not be getting rank soon. Provide us your keywords which affects in rankings so any one can give you proper solutions.