What happens if the supreme court rules congress to be unconstitutional?

Answer 1

Since it is the Supreme Court that decides what is constitutional and what is not, the decisions of the Supreme Court cannot be unconstitutional, however, it is always possible for the Supreme Court to make new decisions which reverse older decisions. So in theory, if the Supreme Court does something wrong, they will be reversed by a later sitting of the same court (but with new judges).

Answer 2

If the Supreme Court of the United States rules an Act of Congress or part thereof unconstitutional, it has no force of law thereafter.

Answer 3

Nothing. Once it is deemed unconstitutional it is no longer valid. A new law can be written and go through the process in congress.