This can happen when Word thinks your text is in a language for which
you do not have proofing tools installed. To fix it, open the document
where this problem occurs, press Ctrl+A to select all of the text, and
click on Tools | Language | Set Language | choose your language |
uncheck the "Detect language automatically" and "Do not check spelling
or grammar" boxes | Default | OK. Then press F7 to rerun spellcheck.

Stressing Student wrote:
I have wrote an essay and for some reason the spell check will no longer
work. If i open a new document the spell check works fine; however if i copy
and paste the essay into this document the spell check will not work