Just to close this up the problem was, as suspected, the QB file was corrupt. In fact all backup versions for this year were also bad. The last good copy seems to be my end of year version, before condensing.    When I talked to support they were not able to offer any tools to fix the file, as might have been hoped. We've got a lot of work to do to recover this situation. Wish us luck.

Thanks to all who made suggestions.