corrupted file FOLDER!

As many of you know, I have sold my graphic shop to friends down the road. But I still 'consult' with them and am teaching them all the tricks. Just ran into a serious problem-they had a last minute order for just a couple 'black and white' 'coverups' to go on political signs. No matter what they did, it wouldn't print. I even came back to my computer (Mac) and redid it exactly the way I always did. No dice. So back up to their shop, and after 2 hours and a dozen tests-even to reloading the software...it suddenly hit me-tests on any other design from other folders worked. Just not from this customer folder! Pulled the design out and dumped it in a new folder-and it worked. Problem? Their FOLDER was corrupted! Just thought I'd pass it on!