How to activate locked iPad?

If your device is Activation Locked, this help page will provide most of the information that you’ll need:

https://support.apple.com/en-us/HT201365

If 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:

https://support.apple.com/en-us/HT201354

More information about recovery of your AppleID password:

https://support.apple.com/en-us/HT201487

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

Which lock, the passcode?

If you forgot the passcode on your iPhone, or your iPhone is disabled - Apple Support

Or the activation lock?

Turn off Activation Lock - Apple Support

Locked in what way? There are numerous ways for an iOS device to be locked. You need to clearly describe what is wrong, and what you have tried to do. With that, someone will be able to help, or at least tell you what you can or cannot do.