If that happens we couldn't get our money back, and this is the same reason as to why many users suggest to not store big amounts  of bitcoins and any other cryptocurrencies on exchanges  if they will not be converted to fiat or to btc.
We can sue exchange owners, staffs or admins case they do that(if their real identities are already revealed).
We can prove how much coins we had in our wallet by using the blockchain technology(specifically block explorers).