If you forgot your username or password, you can simply go to the Password Reset page, enter your email address and request a reset. Your username will be in the email, which will also have a password reset link. Click it, and choose a new password.
If you didn't provide an email during registration and you have a free account, unfortunately there is no way to regain access to the account. If you have a Pro account, open the support bot Garry in the bottom-right corner and describe the access problem. He'll guide you through ownership verification and account recovery. If the case cannot be safely resolved in chat, Garry will offer the next step. The following details will be required: