I was using finder to drop 2k footage from a sandisk 512 gig card when the macbookpro crashed and restarted. When it came too, all the files which were not fully copied to the 2nd drive were missing from the card. The remaining files seem to be fine, though the camera doesn't recognize the card as having any clips on it.
Moral of the story here is "copy", never "move" The behavior you describe can happen for any number of reasons, but the results are usually the same.
You can try a recovery software which might be able to read the SD cards disk structure and possibly recover something.
PhotoRec is free and there are others. SanDisk Rescue Pro (but this costs $$). I'm sure you'll get other recommendaions here.
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Agree with Rick that always copy instead of moving. I would add to his suggestion to talk with someone very familiar with IOS to see what recovery possibilities might exist on the Apple side also.
Another agreement to the two comments above, it sounds like the sudden crash of the Mac may have corrupted some of the memory card data. It is defeinitely a good idea to contact Sandisk to see what recovery options they recommend for their cards, and as above, iOS may have some good tools top assist as well.