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 ar