In Prayer, a Muslim should try his or her best to maintain concentration and humility so as to perform it properly. Reading Surat Al-Fatihah in Prayer is a fundamental part of Prayer without which it is regarded as nullified according to the majority of Muslim scholars.
Dr. Zulfiqar Ali Shah, Member of the Executive Committee of the Fiqh Council of North America and the Religious Director of the Islamic Society of Milwaukee, states: Surat Al-Fatihah is one of the fundamentals of Prayer. The Prophet (peace and blessings be upon him) said: “The person who does not recite Al-Fatihah, there is no Prayer for him or her.”
The hadith is interpreted by various schools of Islamic fiqh to denote different meanings. For instance, Shafi`i and Hanbali jurists interpret the hadith to mean that if a person does not recite Surat Al-Fatihah or misses the recitation of Al-Fatihah in any of the rak`ahs , his or her Prayer is nullified. The Hanafi school interprets the hadith to mean that Al-Fatihah is an extremely important part of the Prayer, but it does not mean that if someone misses reciting Al-Fatihah in any of the rak`ahs, then his Prayer will be nullified. It means that his Prayer will be incomplete, but it will be accepted as a fulfillment of his obligation.
However if one has forgotten or were uncertain whether or not Al-Fatihah was recited, their Prayer is absolutely acceptable and they do not have to repeat it. The Prophet (peace and blessings be upon him) said: “The pen has been lifted (no accounts are written) for three”, and among them is, “someone who has done something out of forgetfulness”.