Hi C4134, thanks for posting this here in Apple Support Communities. We understand that you're unable to locate your Game Center data for a specific game. You've come to the right place, we'd be happy to help. By any chance, is the affected game an Apple Arcade game? If so, and you also use iCloud, we ask that you check out the information within the following resource. More specifically, the "If you don't see your gameplay data on a device" section: Access your Apple Arcade gameplay data on all of your devices - Apple Support
If the game isn't an Apple Arcade game, we'd like to gather some additional information to help determine the best steps to take going forward.
Was iCloud turned off prior to the gameplay data missing or afterwards?
Do you regularly backup your iPhone?
Have you reached out the app's developers yet? This would be a good way to see if the issue is on their end, instead of the iPhone itself.
We look forward to hearing back from you.
Cheers!