What is the ruling on the Ummah if it does something that the Messenger of Allah, may Allah bless him and grant him peace, has forbidden or instituted, and it has not been transmitted from the Companions? Is the Ummah sinful or excused?

This is a hypothetical, unrealistic question, because the religion is preserved, and the Ummah is infallible from agreeing upon error. Therefore, it is impossible for them to concur on permitting what Allah and His Messenger have forbidden, and it is impossible for anything from the religion to be hidden from them. However, some rulings may be hidden from some individuals, and whoever among the scholars strives (to deduce a ruling) and errs, is excused and rewarded.