After changing your AppleID Password, it i necessarily to complete certain tasks on all devices that use the AppleID credentials that have been updated. This support page should provide the information that you’ll need to resolve any issues:
What to do after you change your Apple ID or password - Apple Support
If you switched to using an entirely different AppleID account, your devices may be Activation Locked to the prior account. Here are the Apple support pages that outline the Activation Lock - and the only mechanisms by which it can be turned off or reset.
Activation Lock for iPhone, iPad, and iPod touch - Apple Support
Turn off Activation Lock – Apple Support
If you are the owner of the AppleID that previously activated the device - and you have forgotten your AppleID or associated password, these can be recovered here:
https://iforgot.apple.com
To recover your credentials, you’ll need access to one of the following:
- Your primary email address mailbox that corresponds with your AppleID
- Any of the secondary/recovery email addresses that should be configured for your AppleID account
- Any of the trusted telephone numbers (fixed line or Cellular/Mobile) that are associated with your AppleID
Unless you AppleID account has not been fully/correctly configured, or has been seriously neglected, recovery of the AppleID and password should not be difficult.
More information about recovery of your AppleID:
If you forgot your Apple ID - Apple Support
More information about recovery of your AppleID password:
If you forgot your Apple ID password - Apple Support
When you recover access to your credentials, you would be well advised to log-in to your AppleID account from a web browser - and verify/update any email addresses and trusted telephone numbers:
https://appleid.apple.com