Excel 2007 stops calculating
 

I have a 3gb xslb with about 200 tabs and it has stopped calculating. Many
of the tabs pull data from other tabs, some are static. It had been working
fine for several weeks. Automatic calculation is turned on as is iterations.
Even a manual calc does not force a calculation. Any ideas on how to trace
down a source of the problem? What steps should I take? Thanks.