Unfortunately most casino operators usually have multiple brands and use a technique affectionately called “Cross-Sell”, which means they will sent all players promotions and offers encouraging them to sign up to an alternative brand within the operators portfolio.
I experienced the same issue with my poker database which was over 30K in size with at least 1000 active every month, when the operation had to increase the size of the account number, all of my players disappeared from the back office.
I shared the story a few weeks ago as this same operator wants to work with us on our new casino site. The stats where updated we have $4K of revenue owned but only from 6 players.
So unless you have the players email and ask him to play on the original site again or confirm he is still playing there you are a little stuck.
Not what you want to hear but it is a very common practice. Try to capture your players details as part of a registration so at least you can “Cross sell” them yourself too.