Make sure you copy everything off the card on to a hard drive and try a chkdsk (windows) on the card. If it finds any errors eject the card properly (safely remove hardware in windows). Remount the card and run chkdisk with the fix option. See if the files now show up. And test if new photos are now recognized.
Or format the card in the camera. All you files will "disappear" but they can (mostly) be recovered with a program like Photorec.
Better still is download SDFormatter for the appropriate OS and format the card with Full Erase - this will wipe all you data from the card but will refresh all the logical file structures on the card.
Most people call this a low level format but technically a low level format is done at the hardware level through the hardware controller. Done outside the factory a true low-level format can take hours. The in camera and Quick format will only re-write the Master File Table and not actually wipe the data or re-write the logical file structures.
https://www.sdcard.org/downloads/formatter_4/