How do I unlock my old iCloud account?

Try to recover your old Apple ID password via this link

If you have forgotten your Apple ID password – Apple Support (UK)

If recovering your account is not working, and you have valid proof of purchase information, you can start an Activation Lock support request.

If you have 2 Factor Authentication enabled, if you can't recover the password using the steps above, you will need to use Account Recovery. The Recovery Process is an automated process and the length of time it takes can vary a lot. Apple ID Support can only answer your questions. They can't intervene in or speed up the process. The article below has some suggestions to shorten the time the process takes.

Apple ID - Recover your Apple ID with two-factor authentication account recovery.   *