Each Airport AP has a hidden button on the back to reset the access point.
Airport Extreme: About 1.5 cm to the right of the power supply.
Airport Express: Located 1 cm to the right of the USB port and above the audio port.
If the reset button is pressed (maximum 1 second) the airport is restarted and re-initialized without losing the settings.
If the reset button is pressed for a long time (10 - 15 seconds), the airport is reset to its factory settings.