Welcome to the community, corey316!
I would like to recommend going into Settings>SD Card & Phone storage and seeing if it allows you to unmount the SD card. Then try powering off the phone, removing the SD card for 10 seconds, placing it back in and powering on the device again. That should allow the phone to see the SD card again.
If this is still not working, SanDisk does warranty their cards for five years. You can contact then direct at 866-SANDISK (866-726-3475); between 8:00 AM and 7:00 PM PT, M - F.