Just as what is described in the title, the notebook behaves as if it were originally designed to not have the autocompletion functionality at all. Neither Ctrl+K nor F2 works. Furthermore, reinstalling Mathematica does not help, either.
What measures could be taken that possibly help to recover the autocompletion functionality? I suspect that some items in "Option Inspector" might be responsible but I can not find them.
It seems that quite a few people encounter this problem recently.
But, for now, the "
$UserBaseDirectory deletion / renaming methods" cannot thoroughly settle this issue, can they?