Are you connecting the SD card to a PC? Please connect it to a computer and search for it in Disk Management. Check if it has a correct file format and a drive letter. If the card is missing, search for it in Device Manager. If it is listed there, uninstall it, restart the PC and reconnect the card. Unfortunately, if you have tested the card n several PCs and devices, it is highly possible that the card has been damaged and you may need to replace it.
Hope this helps