Since prodigy no longer exists and is now under AT&Ts control you need to use their password ***** tool which is located here:
If you are unable to use the password ***** tool then I am afraid that your only option is to remember the password.
When you keep an old email address like this with a provider you are no longer subscribed to you run the risk of losing the account especially if you did not set up any back up methods to reset the password ***** as a phone number or alternate email address.
AT&T will only provide support to their paying customers which is why they told you to go to support.att.com which is their way of telling you that you are on your own.
I am hoping you understand that this website is not support.att.com , as you should have noticed this is a paid support website called justanswer.com.
That said, you are going to need to reset the password ***** the link I gave you if you can. If not then you really have no other options except perhaps subscribing to an AT&T service and see if they can get you back in, it is worth giving them a call and asking.
For future reference and to assure nothing like this ever happens again I suggest you always use a free email service like Gmail or Yahoo for your primary email address. You should not use an email address provided by an internet service provider because if you cancel their service or something goes wrong you have no support.
I hope if anything my reply helps you understand your situation a bit more, please take a moment to click the stars above, 3 or more please to acknowledge I have answered you back. You can continue to reply after if you need more guidance.