My iTunes store credit has disappeared after i changed the password

Did you update the email address on your existing iTunes account (i.e. via http://appleid.apple.com) or did you create a new account ? If you did update the existing account and can't see where the balance went (I assume that you've checked the account's purchase history ; my balance has sometimes been out-of-date due to purchases on other devices/computers and logging out and back in, which you would had to had done when changing an account's primary email address, has updated it) : https://www.apple.com/emea/support/itunes/contact.html

You've checked your account's purchase history, you didn't have any auto-renewing subscriptions and/or pre-orders due that could have used the balance : See your purchase history in the iTunes Store on a Mac or PC - Apple Support ? And you've tried logging out and back into the account on the device to see if it a balance then shows, or does it still show a balance on your computer's iTunes ?

This happened to me too. I changed my email because I wanted better access and now my balance is gone. I'm really upset.