Using Samsung with Apple app availabe. I have used it in the past with no problems. WTH?

Go to tv.apple.com and enter your password to see if it works online. If it works when viewing online, then you can look more closely at the TV app and possibly want to do a Factory Reset. If the Password is not working when attempting to watch online, then you have verified you do not have the correct password and will need to reset the password to your Apple ID.

IF you are referring to Apple TV + app then ensure you are using the same login on your TV as you did previously. All, did you you look on the app for a "Forgot Password" or similar function?