Are past prayers considered invalid and must they be repeated if the ghusl did not include water reaching the roots of the hair due to ignorance of the ruling, and what should be done if they are numerous?

The outer body and what is considered externally visible, such as the roots of the hair, must be completely covered with water during the obligatory ghusl. If the questioner limited himself to wiping his hair during ghusl for janabah, then his ghusl is incomplete, and the state of janabah is not lifted unless water reaches the entire body and the roots of the hair. The prayers performed with this incomplete ghusl must be made up to the extent possible, provided it does not harm the body or livelihood. This is the view of the majority of scholars and is issued as a precautionary ruling. However, Shaykh al-Islam Ibn Taymiyyah held the view that whoever misses a prayer or a condition or pillar thereof out of ignorance of its obligation is not required to make it up.