When prompting for the AppleID, it should provide a clue - something like first and last characters with asterisks to indicate hidden letters. Usually, this will be enough of a hint to identify the primary email address that corresponds with the AppleID.
When you’ve worked-out the AppleID used to previously activate the iPad, if the password has been forgotten, it can be recovered here:
https://iforgot.apple.com
Even if you have discarded the primary email address (generally not a wise move, but it happens), you should still be able to receive reset information via one of the secondary/recovery email addresses that you should have created for your AppleID account - or any one of the trusted contact telephone numbers also associated with the AppleID account.
When you have both the AppleID and associated password, you can either use these to release the Activation Lock by entering these details on the device - or you can log-in to your AppleID account and remove the Activation Lock from within the AppleID account pages:
https://appleid.apple.com
Whilst logged-in to the account from a browser, it’s a golden opportunity to verify/update your primary and multiple secondary/recovery email addresses, together with a selection of trusted contact telephone numbers (both fixed line and mobile/cellular numbers are a good idea). Always remember - your AppleID account is the key to everything.
Here is some additional information about the Activation Lock and associated procedures:
https://support.apple.com/en-us/HT201365
I hope this information will provide multiple avenues with which to find a route to resolution of your problem.