Unfortunately, it sounds like you suffered app cache failure and your game cache (including your account information) got wiped.
The only way to get your account back is contacting customer support, which you can find on the Game Menu -> Support -> Contact Us. Try to locate as much information as you have on your original account - purchase history (if any), Player ID, friend code, fighters and levels, etc.
Hope your account recovery is successful, and remember to link it next time!