Trainer,
Thanks for contacting us regarding your sign in issue with Pokémon GO. If you’ve signed into Pokémon GO and it appears that gameplay progress has been reset, be assured that your gameplay progress has NOT been lost. It remains safe and intact. You may have inadvertently created a new Pokémon GO account by unintentionally signing in with a different email address or login method.
If you haven’t already, please try the Pokémon GO Help Center: https://support.pokemongo.nianticlabs.com/hc/en-us/articles/224047008
All Trainers: Keep in mind, there are two login methods for Pokémon GO. Trainers can use their Google Account or their Pokémon Trainer Club account to create a Pokémon GO account. If you create an account with one login method and later attempt to use the other login method, a second account is created. To resolve this issue, log out of your account then log back in using your original login method.
Please note: for security reasons, we’re unable to provide you with the email address you use to sign into your account.
-NianticOps