Hi Bala,
What Scott suggested earlier is correct .. designing a "forgot password" workflow is an application or user experience (UX) issue rather than something specific to MongoDB. It sounds like you already have a general idea of what you want to do (reset the password and send a new one time password by email to the registered address for the account).
Alternatively, you might want to consider using a PHP framework which includes account management features already. An existing framework can save you some time (and heartache) with implementing common functionality. Choice of framework really depends on personal opinion, but as a starting point you could look at those with known MongoDB integration:
http://docs.mongodb.org/ecosystem/drivers/php-libraries/.
If you're stuck on a specific problem using the MongoDB PHP driver, you could post a snippet of code here and we'll try to help.
Regards,
Stephen