بسم الله الرحمن الرحيم
Semen is nijasat-e-ghalizah (heavy impurity) and is in bodily form. If it is smeared with the clothes or body then it is necessary that it is removed from the body and cloth, only then the body and cloth shall turn pure whether it is removed in one attempt or two or three or more. In the above mentioned case, if semen is smeared with your stomach and it was not removed then you were not pure completely and thus your salah as well as of those who offered behind you was invalid. Hence it is wajib to reoffer all those salahs which you offered before washing the semen. When you have performed the fardh acts of the ghusl once, then it is not necessary to repeat the ghusl again, yes the impurity should be washed off.
وكذا يطهر محل نجاسة مرئية بقلعها أي يزوال عينها وأثرها ولو بمرة أو مما فوق ثلاث وفي الأصح وفي الشامية : أي إن لم تزل العين والأثر بالثلاث يزيد عليها (الدر المختار مع الرد 1/ 536 باب الانجاس)
Allah knows Best!
Darul Uloom Deoband