Google recently has rolled out two updates named, Penguin 2.1 and Hummingbird...
We all do one mistake while going for SEO; for us, the search engine == Google, but that's not true. Bing, Yahoo, Ask and lots of other search engines are also there from where we can divert a good amount of traffic as well. While doing SEO, keep mention of at least Bing and Yahoo, apart from Google. It will keep you safe in case of any sudden search ranking drop in Google.
Well, you can't be sure in 48 hrs that the ranking has dropped really; it may get back to original position within a week or so... Therefore don't do anything in hurry. My view, wait for at least one week and keep a keen eye on your daily rankings. Hope, you may acquire a good position again.
If you don't see the site again getting the original position or at least getting near to old rankings then first of all you should check your webmasters account for Manual Webspam Actions (Given in Webmasters Tool). If you see something there then take proper action and apply for the reconsideration.
If there is nothing in Manual Webspam Actions then you should think about disavowing the backlinks. But you should understand the difference between good and bad links - there are some services which you can take help of for the purpose like Link Research Tools. Just remove all the suspicious looking backlinks (contact webmasters to remove backlinks from their sites and if no response from webmasters then disavow that backlink).
After going through this whole process, file an application to Google webspam team for reconsideration. If everything works fine then will again retain your lost position in SERPs.