If you created your account any time before January 15th, 2018, then your data was on the LINE Skullgirls servers.
Unfortunately, we parted ways with LINE and re-launched Skullgirls as part of our own service on January 15th. Due to LINE policy, in order to migrate your data to the new version of the game, you had to create a new Skullgirls Login, otherwise your data would be trapped on their servers forevermore. :(
You can read all about the migration process in our announcement here:
http://forum.skullgirlsmobile.com/threads/official-skullgirls-login-account-migration-faq.2314/
You can also find more information about creating a Skullgirls Login to save your data, here:
http://skullgirls.com/2018/01/skullgirls-mobile-data-loss-migration-warning/
If these steps aren't familiar to you, and you weren't able to create a Skullgirls Login before January 15th, 2018, then we don't have any way to access your data.
If you completed these steps, and you're still having trouble accessing your account, please open a ticket with us so that we can help you find your lost progress!