What should a worshipper do if they recite Surat Al-Fatihah or part of it silently in a loud prayer, or loudly in a silent prayer?

The scholars of the four Madhhabs agreed on the permissibility of both audible and silent recitation in prayer, each in its proper place, even if they differed on whether it is a Sunnah or an obligation. Past prayers in which this was not observed are valid and do not need to be repeated. For future prayers, it is important to maintain this Sunnah, especially since some scholars have deemed it obligatory.