How to reset your AirPort base station?

Soft reset: Resets the base station password and disables security (Access Control and
RADIUS settings) for 5 minutes so that you can join the network to make
changes. If you make no changes within 5 minutes, the base station reverts to
its former settings. Perform this reset if you can't administer the base
station because you forgot its password.

Hard reset: Resets the base station to an unconfigured state, but retains the last saved
configuration and any profiles, allowing you to restore your base station from
previous settings. Perform this reset when the base station is unresponsive, or
you're troubleshooting network issues that aren't resolved by other means.

AirPort Express Setup | Reset AirPort Express | Apple AirPort Express Setup