Depends. Did you have a legal contract with mybitcoin (or anyone for that matter) that said if you deposit the coins you would get them back? Or... Did you just send your coins to an unknown party and hope that they would be nice and reciprocate by returning your coins? If you don't have a legal contract, perhaps you just gifted your coins away.

If you are looking for legal remedy, you have to specify what laws or contracts were broken. I don't see any laws or contracts broken.

I realize that this might not be the moral answer, but it is probably the legal answer. I am not a lawyer so take it for what it is.