السلام عليكم I was praying fajr finished my first rakah, my second rakah was interrupted while reciting Fatiah which lead me to forgetting where I was ,so I said Ameen and went with my salah. Due to the hadith in the meaning of praying 1 rakah suffices the obligation, is the obligation sufficed?

If you have not recited the Fatiha in the second rak'ah, or you lost your place, what you do is you can start over from the place that you know you have said. If you have already said the first ayah or the second ayah, you can start from there; that is no problem. However, if you did not recite Surah Al-Fatiha, then you must repeat the Salah.