I understand Coinbase uses its own addresses (rather than individual user addresses) so as to be able to aggregate transactions and thereby reduce transaction costs. But the address should be under their control, so my suggestion would be to contact them. Was the address that sent you the coins 1 address with a very high balance? If so, it's likely to be one of Coinbase's own addresses, and in that case they should be able to give you back your coins (if they're willing to do so).Another possibility is that Coinbase sends funds from other user's deposit addresses. If that's the case, your funds were credited to someone else's account, and in that case it's unlikely they can give it back to you.
It depends on the way Coinbase handles withdrawals.