You cannot retrieve the password from the AirPort - not even if you could use AirPort Utility - but it should still be stored in your Keychain. If you "hard reset" the AirPort all its settings will be erased, and you would need to configure it again.
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 Extreme, one that is used to connect to your Airport Extreme 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 Extreme 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).
(See Note below though)
If you are trying to recover the AirPort Extreme 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.
Note: The WPA Pre-shared key may not be called "AirPort Network Password". I never used one of those so I am not certain what name Keychain uses, but you ought to be able to find it in a manner similar to the above.