Mss won't generate T of C

["Barrie Fairley" <barfair () comcast ! net>]

I have a manuscript that I am working on and previously saved versions
readily permitted the generation of a Table of Contents.  The latest version
of the mss will not.  When I click on Tools |Table of Contents | Generate,
NB goes in to the 'Program not Responding' mode from which I have to exit by
Ctrl+Alt+Del.  

What can happen to a document that would cause this?  Is there any way I can
fix this without having to go back to a previously saved version?

Barrie.