If you don't have access to any, only bitcoin address where you can sign a message from. Just write to them about why you can't sign a message too. A staked address is not a must but increases your chances of having the account recovered. What is a must is access to the original email address you used for registering.

They will also look at the IP logs of your account, before and after you lost access to it. That will be used to confirm that indeed your account changed hands.