Paste this in your search Engine and see.
There's nothing much outsiders here can help you except try to provide information with you. If indeed it's located in Poland. You may need a lot of money and legal work to track them down. It would have been much easier if you were in Poland and working with the local authorities there.
If OP will decide to contact legal people, they need to prepare for the money to pay them and the waiting time if ever they will resolve this situation. Also, make sure that they have all the documents that they need to present a strong case against this site. By the way, is this Polish exchange by any means registered under Poland jurisdiction? If not, then, it would be very hard to run after them, and maybe they will just be wasting their resources.
I have a feeling that it will be very difficult to run after this exchanges.
And yes it will take a lot of time and money and we don't know what will be the outcome. Then we have the question of jurisdiction, there's a lot of loopholes that it's going to be hard, unless the Polish authority themselves chases this exchanges and order them to pay back its customers. Other than that, I don't think someone here can help the OP out, just saying.