For those not using some password software that automatically saves your passwords - I would encourage you to use Keychain Access for this.
Whenever you're going to create a new password, open up Keychain, click on Edit in the menu bar and then New Secure Note Item. Create your new item, then copy & paste from the item into the username / password fields you are setting up. In this way there is no doubt that you have a correct backup for all your log ins.
For your immediate dilemma - if you had 'Remember password in Keychain ' checked, it will be there. If you didn't, it's time to change the password and easily done - when you go into Airport Utility, click the Manual button.