In your paper, press Ctrl+A to select all of the text, click on Tools |
Language | Set Language | choose your language | uncheck the "Do not
check spelling or grammar" box | uncheck the "Detect language
automatically" box | Default | OK.

Adrian wrote:

I am currently writing an research paper and I noticed
that the auto-correction stopped working at the 5th page
of my paper. It does not underline mispelled words or
fragments. Also, I cannot look up synomyms for any word.
Can anyone help!