Answer:

In the name of Allah, the Most Gracious, the Most Merciful.

To understand what happens if someone forgot Surah Fatihah in prayer, we must clearly distinguish between two completely different situations: praying individually (or leading as an Imam) versus praying as a follower behind an Imam. The Shariah has prescribed distinct rulings for both.

1. Praying Individually or as an Imam:

Reciting Surah Al-Fatihah is a fundamental pillar (Rukn). If a person is praying alone or leading the prayer and completely forgets to recite it, that specific Rak'ah is invalid. This is based on the Hadith: "There is no prayer for the one who does not recite the Opening of the Book." [Sahih al-Bukhari: 756]

2. Praying as a Follower Behind an Imam:

The ruling changes entirely when you are praying behind an Imam. According to the Holy Quran and authentic Sunnah (followed by the Hanafi school and a vast majority of scholars), the follower (Muqtadi) is commanded to remain completely silent while the Imam recites.

This is explicitly clarified by the Prophet Muhammad (ﷺ) in Sahih Muslim. The Imam is appointed to be followed, and his recitation suffices for the entire congregation.

Furthermore, another well-known Hadith states: "Whoever has an Imam, the recitation of the Imam is his recitation." [Sunan Ibn Majah: 850]

Therefore, if you are praying behind an Imam, you do not need to recite Surah Al-Fatihah. Consequently, the question of "forgetting" it behind the Imam does not invalidate your prayer. Your prayer is perfectly valid through the Imam's recitation.

This beautiful distinction prevents confusion in the Ummah and preserves the harmony of the congregation. And Allah knows best.