Before doing anything else, run a ChkDsk on the card from your PC.
1. open a command line prompt on your PC.
2. type in "ChkDsk" followed by the drive letter of your SD card.
If you are lucky, you will be able to "fix" the card.
If you are successful, then retrieve the files from the card, then format before using it again.
The best thing to do is to do a long format from the PC. Many cameras do a quick format, but some Canon cameras do what they call a "low level" format, which is in reality the same as the long format in the PC.
Doing a long/low level format will ensure any corruption that the ChkDsk did not find will be repaired so you can use the card again.
Also, if the card is going kaput, the ChkDsk should detect it.