How do I get back into my old Apple ID

Did you forget the ID/password?

Apple ID - How to find your Apple https://support.apple.com/en-us/HT201354 An iCloud e-mail address is an Apple ID.

Apple ID - Find https://iforgot.apple.com/appleid#!&section=appleid

Enter the Apple (iCloud) ID and click Forgot Password.

Apple ID - Manage https://appleid.apple.com/#!&page=signin

Apple ID – Forgot Password https://support.apple.com/en-us/HT201487

Did you change your Apple ID. Or, do your have two of them. In the first case, you don't get back into it. In the second case, you have to sign in to the one your want to use.