the Imam forgets an Ayah from Surah Al-Fatihah, and does not remember until a long time afterwards, he should repeat the Salah, if it is a Faridah (obligatory Salah). This is because reciting Al-Fatihah is a Rukn (integral pillar) of the Salah. The Prophet (peace be upon him) said:
“No Salah (Prayer) for the one who does not recite Fatiha-tul-Kitab (the Opening of the Book, i.e. Al-Fatihah).”
However, if he remembers it shortly after his Salah, then he can offer another Rak‘ah (unit of Prayer), instead of the Rak‘ah in which he forgot to recite an Ayah from Surah Al-Fatihah, then performs Sujud-ul-Sahw (Prostration of Forgetfulness).
If the Ayah forgotten is not from Surah Al-Fatihah, then his Salah is correct, and neither he nor the Ma’mums (persons being led by an Imam in Prayer) need to do anything,
because reciting more than Surah Al-Fatihah is Mustahab (desirable) and not Wajib (obligatory).May Allah grant us success. May peace and blessings be upon our Prophet Muhammad, his family, and Companions.