Lets say you can retrieve password based on username. Will send password in an email registered with that username.
Your UI must act something like, when 'Forget password' pressed ask for the username. When username supplied, check if it exists. If so, retrieve the password and send across to the email registered against it. Flash a message that you did this.
How to sent Forgot Password in Mail[
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This was simple one, you can ask for new passwords, etc - it depends on the complexity/security you prefer for your website.