Board Thread:Love Live! School Idol Festival Lounge/@comment-26545383-20150906073707/@comment-24187738-20150906074754

You might be able to find your ID if you knew any of your in-game friends. Otherwise it's pretty much impossible.

You should try to provide them with as much details as you can (if you ever took any screenshots you could refer to them I suppose), but if you're unable to do so, the chances of getting back your account are very low.