You can retrieve your existing wireless network password from a Mac you normally use on that network.
Read the following to determine how to retrieve your password from Keychain:
Your wireless network password is stored in your Mac's Keychain.
You must first have an Administrator account on a Mac that you use with that AirPort Base Station, one that is used to connect to your AirPort network.
On that Mac, open the Utilities folder. Find Keychain Access and launch it.
In the search field at the upper right, type Airport. All the Keychain entries for your base station will appear there, as well as other entries.
Look at the column "Kind". The entry you want to select will be labeled "AirPort network password" (not "AirPort base station password" or anything else).
If you are trying to recover the AirPort Base Station password then the entry you want to select will be labeled "AirPort base station password" not "AirPort network password".
Double-click that, and in the window that appears check the box for "Show password".
You will get the following window:
In the Password field type your Mac's Administrator password - the same one you used to log in to that Mac.
Then, click Allow, and your wireless network password will appear in the "Show password" field:
Make note of it.
Close the window, then quit Keychain Access.