What version of Word?
It sounds like the text has acquired the "Do not check spelling or grammar"
settings.
Select all (Ctrl+A), either double clcik the Language button at the bottom
of the Word window, or Tools, Language, Set Language, clear the "Do not check
spelling or grammar" box, while here also clear the Auto Detect Language,
wont hurt.
If the "Do not check" was not checked, try Tools, Options, Spelling &
Grammar, check both boxes for the Spelling and Grammar "check as you type".
The above is for 2003 and previous, 2007 has slightly different layout but
you will be looking for essentially the same commands.
Hope this helps, if not come back.
DeanH