You should follow these solutions to fix blank SD card error:
Solution 1: Switch OFF and then Switch ON your phone or reboot your phone and then check if it is solved or not.
Solution 2: If above solution doesn’t work, then remove the card from the phone, clean it – remove the dust particles, particularly the area of metal parts and then re-insert the card into the phone and check.
Solution 3: If still your problem is not solved and you do not want to lose data from the card then in that case I would recommend you to first use Photo Recovery software, it will help you to retrieve photos, videos and audio files from the card. After that you can format your card to re-use it again.