Hi Aj,
Have you formatted your SD card? SD cards need to be formatted before usage to ensure compatibility with the camera. Formatting will delete any content on the card, so make sure to transfer all of your content from your SD card to a computer.
With your SD card inserted in the camera, turn your camera on. Press the "Menu" button. Select the wrench icon and then "Format."
If this still doesn't solve the issue, take a look at the Canon Rebel T3i [[Canon EOS Rebel T3i Troubleshooting#Section_Memory_card_recognition|troubleshooting page]].
Best of luck!