I've done a number of hard drive and SD card recoveries in my days. While I am no expert by any means, there is one program that has worked on hard drives and flash memory where the half dozen others I tried failed miserably. Note: I am in NO way associated with the author or owner of this software, its just that I've used it successfully a few times and bought licenses. One particular laptop hard drive was trodden on and nothing else even came close, but this managed to get quite some of their data back.
I'd highly recommend that you lock the SD card before plugging it in to your PC, so it can only be READ and not modified by this or any other software, that way something else might work if the other fails.
This is it -
About DMDE (DM Disk Editor and Data Recovery Software)
Anyway.. download and have a look and get the "GUI for Windows" demo (which is free) - it will recover a single file in demo mode to prove it works, then if it does you can purchase a license to get all the files. Its a bit convoluted to buy as it registers to your system, but worth it at $20 IMO.