I'm sure you've already tried restarting your phone by holding down the power and volume down buttons together for about 10 seconds until it vibrates, right?
If so, the next thing to try is to do a remove and re-seat. Go to Storage Sense (in settings), then tap on your SD card, scroll all the way to the bottom, and tap "remove card"... Remove it, then reinsert it. See what happens...
If you're still having the issues after that, you may need to reformat the card, which you may have seen where to do that right next to the "remove card" button.
Let us know how it goes..