There are various ways to re-obtain German citizenship. For example, Sec. 13 of the German Nationality Act provides a facilitated process through which former citizens may seek re-instatement.

However, that procedure is only available if an applicant can demonstrate that he or she (a) would have been granted a Retention Certificate (had a timely application been made), and (b) maintains sufficient ties to Germany that still exist today. A reinstatement application must be made within 12 years of loss of German citizenship. “Read more about dual citizenship and re-obtaining German citizenship here”