Since Office 2007 is beta, there isn't a lot of experience yet with problems
such as disappearing ribbons. First, I would try starting Word 2007 in Safe
Mode. Hold the Ctrl key as Word is starting, then say Yes to safe mode. Does
the ribbon come back?
If the ribbon comes back, then the problem likely is a corrupted registry
entry or a corrupted normal.dotm (or a corrupted add-in of some kind). You
might try adapting the steps shown here for Word 2007:
http://word.mvps.org/FAQs/AppErrors/ProbsOpeningWord.htm
Note that despite the fact that this is named "Problems Opening Word," it's
really much broader than that. Some of the same issues that can prevent Word
from opening properly can cause other problems as well.
Note that a technical refresh patch is due soon for Office 2007 Beta 2, and
applying the patch might fix the problem as well. Check the Beta download
site every day or two to see if the patch has been posted yet.