You are going to have to find some kind of information that only the true account owner would know and will convince Apple you aren't a hacker trying to get into the account. Unfortunately people occasionally are somewhat lax about providing information they can remember and the consequence is the account is now forfeit.
Several years ago Apple transitioned to using two factor authentication instead of security questions. You can continue to use security questions if you already use them and they are working for you, but if you have problems then they no longer get direct support. You can try the directions in this support article If you forgot the answers to your Apple ID security questions - Apple Support but the article is labeled "archived" and may contain information that no longer applies. If the instructions work then fine; if they do not (e.g., you see “not enough information”*) then you will have no option but to change to the newer security method, if you can. Over the past few years Apple has been persistently encouraging users to switch to the newer two factor authentication security system. For those who have disregarded their encouraging, Apple will likely only help you to now change to two factor authentication, if even that is possible. Follow the instructions in the "Turn on two-factor authentication for your Apple ID" section in this support article --> Two-factor authentication for Apple Account - Apple Support to try to change to two factor authentication.
If you still need help, contact Apple Account support at this link ➞ https://getsupport.apple.com/?caller=cups&PGF=PGF63005
Select "Other Apple Account Topics", then "Security concerns or account compromised." This will give you a chat or telephone call option.
If Apple cannot, or will not, help you transition to two factor authentication then it is quite possible you will have to simply abandon the old account and everything it contained. In that case, if you were using the Find My service, the Activation Lock support request support article at: - https://al-support.apple.com/#/additional-support may at least let you regain use of your equipment. If not, you will also have to say goodbye to the device as well.
Good luck!
