Is your card accessible VIA explorer? Or can the device be managed in disk management? If the above 2 cannot be performed, the card have likely suffered "boot" sector corruption.
I should be able to help but without 100% guarantee. As long as the device can be low level access, you've chance of recovering those files, but if the device is electronically corrupted, you'll have to pray really hard.
Let me know how does this happen and I might gauge the chance.
This is what I encountered previously;
1. Card suddenly disappeared from card reader and the card reader also disappeared in explorer.
Chance: >80% pic recovered, with other being corrupted.
2. Device suddenly became very slow and always have timeout, unplug and plug it back, Windows said the device is not formatted and need to format it. (Obviously I'm not stupid to format that. :))
Chance: Nil, recovered files are all garbage.