Actually "something is broken" is and should be the correct answer. There is nothing in English like (something broken) without any "to be" verb connecting the adjective to the subject before it.
Another translation for شيء مكسور could be: a broken thing. The Arabic sentence would accepts both translations but it depends on the whole context to decide which English translation to use. But here, there is no context but a simple phrase, and both English sentences should be accepted (but not "something broken").