Most likely the file has been corrupted by disk problems.
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Go to your backups ASAP and make copies of the file from there, so as to not chance them being overwritten by this corrupted version.
You did make backups, right?
Yes, Time Machine has them (I hope so very much!!
Yes, the backups did work. I lost a week of work, but not the whole picture (whew!) No question the file ws corrupted, because I found two other psb files in the archives which did open. I think the corruption happened from letting the file stay open while the machine slept - trying to save the start-up time.. Never do that again. Thank you both. The solution was not the easy one, the no pain one I was hoping for, but it did work.