If the SD card is formatted as internal that could well be the problem. Try and copy all personal data, photos etc. to another cource, a cloud or better a PC.
When you check the Settings > Storage if the SD card is Portable you will see an eject icon to the right.
Formatting as internal means data on there is encrypted so you cannot retrieve it in another device. Whereras formatted as Portable/External removes lots of potential problems and means you can access the contents elsewhere.
Look at the images shown in this post