Ok, check to make sure the card isn't corrupt. 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.
The card could also be counterfeit, meaning it's actually a smaller size than it claims. Once the phone tries to save more to the card than it can actually handle, files can get corrupted.
SD Insight might help, but it can have problems working on Nougat or above.