Most modern routers allow you to backup the configuration of the router
into a file, and then restore the configuration from the file when it's
needed.
The backup file of the router usually contains important data like your
ISP user name/password, the login password of the router, and wireless
network keys.
If you lost one of these password/keys, but you still have a backup file
of your router configuration, RouterPassView might help you to recover
your lost password from your router file.