wifi only iPad mini won't recognise apple id password after 8.4.1 upgrade

I backed up to my iMac and updated my daughter's wifi only iPad mini to iOS 8.4.1 yesterday and since then we cannot get FaceTime or any thing else that requires the the apple id and password to be entered as it does not recognise the password. i have changed the password and it is working on all other devices (iMac, 2 x iPhone 6) but not on the iPad mini.

Please can anyone help? never had this issue before i updated the iPad.

I have a very upset daughter!