I would try checking the SD card in a pc to check the contents are readable, then I'd test it using h2testw software for any corruptions.
After passing the test, I'd then back it up onto the pc, once done, format it, then put it in phone.
Once in phone go to settings, storage, tap the SD card, then the three dots in top right, storage settings.
Then there will be a button that says format.
Do it once more in phone and that should work fine going forward.
