Welcome to Android Central! The SD card could be corrupt, defective, or counterfeit. Install
SD Insight to see if the card is genuine or counterfeit. Counterfeit cards are programmed to report more storage than they actually have, and if you try saving more than the card can actually hold, then files can start getting corrupt. (Be aware that SD Insight might not work on all Nougat or above devices.)
If it's a valid card, then go to
Settings>Storage, Unmount the card, remove it, and insert it into your computer. Can the computer read these files? Backup as much as you can right now, then run chkdsk to look for bad sectors:
http://forums.androidcentral.com/amb...d-sd-card.html.
