I have an Airport Extreme base station running on OS X Lion. The base station also serves as a kind of "time capsule" by having a 500GB hard drive connected to it.
It had worked perfectly until a couple of days when the base station "disappeared" from my network connection in Finder, all my music is saved on this disk and it can not interact anymore with my computer.
I tried changing the settings of the base station, but it now seems that I no longer have access to it with the password I used to work with.
I would start with a "soft" reset. With this type of reset, the AirPort's passwords will be temporarily disabled for up to five minutes. This should provide you with adequate time to use the AirPort Utility to change it.