Sometimes you lose your master password. Won't describe here how you managed to do it, since distraction that lead to that kind of errors is a burden to carry for all of us.
Drupal 6 was quite easy to recover, because its passwords were encrypted using the well known MD5 digest algorithm, while Drupal 7 uses SHA512 and a custom seed depending on the site. This statement makes us unable to use well known functions for that.
In order to recover your password, there are two easy steps.