Eeish Stephen, that is not fun
Try sandisk rescue pro. as you are using Sandisks, you'll either have a tiny disk ( that wont fit into your laptop) or a serial number for the program and a link to download.
Disk utility.app, that may work for you as you are on a mac like I am
There are many free and paid for apps for mac and PC that allow you to do rescue files from disk.
I've formatted chips in my camera, prior to taking photos off several times, and after saying a few BAD WORDS taken that chip out , shot with another chip and once home I've used sandisk rescue pro and got all the photos back.
Stating the obvious, but dont reshoot using the suspect chip before trying to rescue, as that would over write of course.
Of course, if they are corrupted, then you may be using BAD WORDS as well.
I'd do a test with another card, and if that card is fine, repeat the test with the suspect card, and if that card breaks, use it as a coaster.
I hope that works for you.
Thanks Hamish, I'll give the rescue disk a try.
My concern is that something in LR4 caused the error, given that I've never seen it before. Assuming that Adobe haven't changed the import routines in LR4, it may just have been a coincidence.
Given that the files were all corrupt on input, it could be that the card was failing badly, and eventually failed to the point that it appears empty. Like Hamish says, that card may be coaster material.