回事 almost never stands alone. It follows a number or a word like 怎么 or 这么: 怎么回事 'what's going on?', 这么回事 'that's how it is', 一回事 'the same thing', 两回事 'two different things'.
grammar
当回事 means 'to take seriously', with 当 read dàng, 'to treat as'. It usually comes with 把 or a negative: 别把它当回事 'don't take it to heart', 不当回事 'to shrug something off'.
