The protected FRITZ!Box user interface cannot be opened without a password. If you have forgotten your password, you must restore the factory settings of FRITZ!Box. For the first 10 minutes of FRITZ!Box operation, the option "Restore Factory Settings" is available even without entering a password.
Attention: This operation will discard all of your settings.
Unplug FRITZ!Box from the power mains. After a minute or so you can access the FRITZ!Box user interface again and restore the factory settings.