From time to time a user may have disabled Word’s built in proofing (spelling and grammar checking).
To re-enable it, highlight all the words in the document, goto the REVIEW tab and select Set Language.
You will likely see a checked checkbox labeled Do not check spelling or grammar. Removing this check will re-enable the proofing on this document only.
To re-enable for all new documents by default, click on the button labeled Default..
Click YES and you’re done!