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Hi, thank you for contacting JustAnswer.com. My name is Russell. I will do my best to provide the right answer to your question.
If the particular WiFi network has had its WiFi passphrase (commonly known as the 'WiFi Password') changed, then if your iPhone has a stored passphrase for it, that stored passphrase is no longer valid.
I suggest that you try joining the WiFi network in question, as if it were entirely new to you and new to the iPhone (if you chose 'Forget this network' for this WiFi network, then it is once again new to the iPhone). That might allow the right login/password/passphrase to work properly for the WiFi once again.
Let me know more about the problem, of course, and then I will have more advice to offer.