Just changed my Apple ID password on my iPhone but my iPad will not accept the new password. Please advise?

Click on the link and read it to learn "What to do after [or before] you change your Apple ID [email address] or password" --> https://support.apple.com/HT204071 - "After you change your Apple ID email address or password, you might need to update your sign-in information with any Apple services you're using." - Or if you're seeing persistent disabled messages or repeated requests for the password to your previous Apple ID. You may also wish to turn off "Find My" before changing information and turn it on again afterward.

Some of the settings on the iPad have stored the old password and are still trying to use it. Changing it on the iPhone told Apple's servers to update it to the new one but the iPad is still trying to use it to connect to them and being rejected. Use the link I provided to guide you in changing it on the iPad too so it starts using the new password. It makes sense that changing it one one device does not automatically do it on all others. Let's say your iPad was stolen and you used your phone to change your Apple ID password. It would prevent people using the stolen iPad from buying things with it.

Sign out and then try a forced restart.

Reset Device.   

Have you tried signing out of the iPad, restarting the iPad, and then tried to sign back in?