Site ranking performance (SERP) on Google may suddenly drop or the site may no longer appear in Google results for some unknown reason (where the Webmaster does not believe that the site violates any Google Webmaster Guidelines). In these cases, the webmaster may submit a reconsideration request to Google. Also, if a site was penalised for violating the Google Webmaster Guidelines but the issues relating to the penalty were resolved, then the Webmaster also can submit a reconsideration request.
It is important to be as detailed as possible when filling out the request form. Read the guidelines, admit to mistakes and advise on actions taken, include evidence and don't spam the request. Be patient, reconsideration requests can take some time to process.