The Verdict: Need More Info
This is a descent question: it turns on whether your parent was still German when you were born. Take the Germany Citizenship by Descent quiz, or ask the Bundesverwaltungsamt for a certificate of nationality.
The old loss rule was personal. It took citizenship from the person who acquired another nationality, not from a child already born. If your parent naturalised abroad after your birth, you were born German and probably still are, and you need a certificate, not an application. If before, you were not born German, and the route for former Germans does not apply.
A child who gained a foreign nationality automatically when a parent naturalised generally did not lose German citizenship, even though the parent did.
Gather your full birth certificate, your parents' marriage certificate, your German parent's birth certificate and any foreign naturalisation file; its date usually decides the question. Holding no German passport proves nothing, because citizenship passes automatically.
Two routes can reopen a line that looks closed: restoration, if the family left Germany between 1933 and 1945 because of persecution, and a declaration route open until 19 August 2031 for lines through a German woman who married a foreigner before 1975.