If you acquired U.S. Citizenship after January 1, 2000 and lost your German citizenship because of that, you might still get your German citizenship back.
German citizens who apply for and acquire US citizenship without a valid “Beibehaltungsgenehmigung” (retention certificate) in hand, will lose their German citizenship. Fortunately, if one inadvertently loses his or her original citizenship, Germany has procedures to re-establish it. Actually, the relevant procedure has been in place for quite some time. It is set forth in § 13 of the German Nationality Act (“StAG” or “Staatsangehörigkeitsgesetz”). However, that procedure is only available if an applicant can demonstrate that he or she (a.) would have been granted a Retention Certificate (assuming that a timely application had been made before US citizenship was acquired), (b.) the returning citizen continues to maintain sufficient ties to Germany, and (c.) the returning citizen has the financial resources to provide for his or her own maintenance.
Notably, until late 2019, for § 13 StAG to apply, the inadvertent citizenship loss must have occurred within 12 years prior to application of the procedure. However, this deadline has been changed. Now, the loss may have occurred on or after January 1, 2000. This positive change opens up applications for many former German citizens to get their German citizenship back.