Comcast xfinity email will not open

From what you are describing, you are directly accessing the Comcast web site on the internet with a browser. If that is correct, your Mac is involved only so far as to provide the web's password as it is stored in your keychain. You seem to be saying that the second access to Comcast email links is not accepting your password. My experience in accessing the Comcast web site is that it seems to have the passwords stored there for its email access link(s), which are accessed after the initial Comcast access. If I am on track here, you need to go further on the web sit and reconfirm what passwords they have stored for you. You can also contact Comcast for assistance with your access problem.

We need a bit more clarity. What message are you getting? Is this happening with trying to open your email app or when going directly to Comcast's web site? It sounds like you may have to reenter your password(s) there.