Suddenly phone stopped accepting my Apple ID & pass

After owning and using my iPhone 6 for as long as the 6 has been out .. one day while filling out forms on line... suddenly my phone requested my Apple ID password .. I was not doing anything thing to prompt that pop up.. so I canceled it and returned to my forms.. I was on a gov web site..

an hour later it came up again and continued to pop up..

so I entered my password and the phone said it was incorrect !

so I went to my PC and did the same.. and the password worked .. so I tried again with my phone which should have worked since it’s the same Apple ID .. and the phone locked me out and now it requires entry of the Apple ID which now the phone says is incorrect !

but when I use FIND MY ... the phone devices listed is my iPhone 6 ... so I do not understand what the malfunction or where to begin resolving this problem