Assuming that the file system on the card is trashed (second worst case - the worst case id that the card's been ground up), download
PhotoRec to your PC (Windows, Linux or Mac - there are versions for each). Instructions are at
PhotoRec Step By Step. It's going to take time, but if the card isn't physically damaged, PhotoRec will recover every file on it, accessible or not. (It looks for the files themselves, not for their entries in the directory.)
It can take days, depending on the size of the card, but if PhotoRec can't recover files, they can't be recovered. (And they don't get copies made in the phone unless you copied them.)
As GameDus said,
DO NOT do anything to the SD card, except remove it from the phone (either eject it [which doesn't
physically eject it] or turn the phone off - then remove it.
Deleted picture apps write to the card. PhotoRec recovers files to any other drive, but not to the one it's recovering from. It's forensic software, that means that the original
must be left intact. (Yes, it should sell for hundreds of dollars - it doesn't, Christophe Grenier, the developer, has kept it free.)