How to retrieve a Pokemon Go Account
If You used a password, you most likely signed up in Trainer Club". Google+ doesn't require a password.
Follow this link: https://club.pokemon.com/us/pokemon-trainer-club/forgot-password
Make sure you select "request desktop site" in your tools menu in your browser. If you don't, you will not be able to reset password from mobile phone.
Enter your email address you signed up with and a email will be sent to you. At the bottom of that email, click on the link "Reset Your Password".