Your CyberGhost account relies on your user credentials. Once you lose those, you can’t access the CyberGhost VPN network anymore. A possible subscription will be gone as well.
For safety reasons you got the PUK, your ‘Personal Unblocking Key’, which has been generated when you opened your account and which has been (hopefully) saved by you somewhere. With this PUK you can reactivate your account in case you lost your credentials.
Will the PUK be able to compromise your anonymity? No! The PUK is bound to your account, that’s true, but your account is completely anonymous and the PUK is bound to nothing else. No one but you has a copy of that key (not even CyberGhost) and therefore nobody can use it to unveil your identity or takeover your account. The downside: It’s solely you who’s responsible for your PUK, and not even CyberGhost can help you out, if you lose both, your credentials and your unblocking key.