Is the data source being moved or renamed? If not, it's possible the mailmerge main document is corrupt. Corrupt documents can often be 'repaired' by inserting a new, empty, paragraph at the very end, copying everything except that new paragraph to a new document based on the same template, closing the old document and saving the new one over it. You will need to re-establish the mailmerge links to the data source as well, as the new document won't initially be a mailmerge main document.

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Cheers,
Paul Edstein
[Fmr MS MVP - Word]