If you registered your account and don't remember your password, check this article: I want to change my password.
If you don't have the access to the e-mail address you registered your account under, you won't be able to reset your password. If you need to change the e-mail address, please check: I want to change my e-mail.
If your account wasn't registered and the game data has been wiped or reset on your device, we may not be able to restore it.
Make sure you register your account and use a valid e-mail address during the registration process!