Cannot Sign in AWS account

The email address that received the email from AWS is very likely going to be the one that you need to sign in with. If you have forgotten the password that is used with it, this can be reset by following these steps https://docs.aws.amazon.com/IAM/latest/UserGuide/id_credentials_access-keys_retrieve.html#reset-root-password

To stop any future charges after free tier ends please review these steps here https://repost.aws/knowledge-center/stop-future-free-tier-charges

A more extreme method is using AWS Nuke (caution! use with care!) https://docs.aws.amazon.com/prescriptive-guidance/latest/patterns/automate-deletion-of-aws-resources-by-using-aws-nuke.html

If you are still unable to login then use this form to contact AWS Support https://support.aws.amazon.com/#/contacts/aws-account-support/