Must Fajr prayer be repeated if the worshipper discovers he was in a state of ritual impurity (janabah) after the semen had dried and he did not feel any wetness, and the time for prayer has expired?

If you are certain of ritual impurity (janabah), then ritual bathing (ghusl) is obligatory, and you must repeat the prayers you performed while in a state of janabah, even if their time has expired. However, if it is merely a doubt, do not pay attention to it.