If you are able to see the Photos on your PC, you can copy those. No recovery procedure may be needed.
Once you have ensured that you lose nothing, try to format the card in the camera, take a few shots and check whether you can see it in the camera.
You may also choose to format the SD Card with SD formatter
https://www.sdcard.org/downloads/formatter_3/
My thinking is that once you format the cards afresh, your camera should be able to store and display the photos.
Now the theory:
The data stored in flash memory is not permanent. The data is stored as charges and over a period of time the charge dissipates. The data then can become unreadable. As always, some better readers can still read them but sooner or later those will also be not able to read when more charge dissipates.
On the subject of long-term data storage on flash memory, it is always recommended to periodically copy the data to another media, format the storage device and rewrite the data afresh.