Since you haven't bothered to tell what version or update level you're
using, let alone anything about the OS or your Mac, about the only
suggestion to be offered is to have a look at the standard troubleshooting
procedures detailed here - specifically those for "Unexpected quit while in
use":
http://word.mvps.org/mac/Diagnose.html
If these steps don't resolve the problem feel free to post back with your
results, but *please* include enough detail for someone to at least know
whether you're even using a Mac