You should go to iCloud to see if your lost files are backed up previously or restore the files from the backup if you have one. If there is neither, then it's a pity that the files are all gone away and you have no chance to recover them back.
Take care~
Guide to resolve problems when Disk Utility won’t let you erase hard drives or SD cards because the erase button is grayed out or the “Erase process has failed” error pops up.
This is a complete introduction to SDHC cards, a type of SD card. It elaborates on the SDHC card's multiple aspects, including size, capacity, compatibility, file system, cost, etc.