This is a pretty common problem for many users returning to xat, but there is a simple answer!
You'll need to open a ticket to recover your account. To do so, make a new account. Instructions regarding making a new account can be found here. Once you make the new account (on a new email), go to xat.com/ticket and open one under the department: 'Lost Access'. When opening the ticket, please ensure the subject is five words or longer. Something like, 'Help I lost access to my account', should suffice.
The ticket times page suggests there aren't any delays in this department, so you should receive a reply relatively quickly (probably under five days). You may be asked some questions regarding the account to confirm you're the original owner. Just so you know, all passwords were reset after the data breach, so this is absolutely necessary.
If you experience any issues, feel free to reply on the thread.