Possible solutions:

1. A missing toolbar may have been accidentally hidden. Right-click a toolbar and pick the hidden one.

2. In Word 2000 (and over) try start > run > winword.exe /a

3. Delete the registry key to reset values to default

DISCLAIMER: as always, registry editing has the potential to damage your PC. I take no responsibility for problems caused by following this path. If you do try this, make sure you back up your registry first, and make a note of the values within the key you want to reset before deleting it.

For Word 97 the key is...
\\HKEY_CURRENT_USER\Software\Microsoft\Office\8.0\Word\Data\Toolbars

For Word 2000 the key is...
\\HKEY_CURRENT_USER\Software\Microsoft\Office\9.0\Word\Data\Toolbars

For Word 2002 the key is...
\\HKEY_CURRENT_USER\Software\Microsoft\Office\10.0\Word\Data\Toolbars

Deleting these will reset toolbars to factory defaults once Word is restarted.

If your toolbars are ok but other settings are corrupt, do as above but delete the 'Settings' key in ...Word\Data\ instead.