It sounds like your SD card got corrupted or ejected. It seems Android can't read or write to the SD card.
You can try mounting the SD-card again. To do this, go to Settings -> Storage. Many devices will show an option to mount the SD card. If your device has this option, press it and try again.
If that option is not available on your device (or you get an error message), your SD card might be corrupted. Try inserting it into another device (PC, smart phone, etc.) and see if the files are readable. If they're not, your SD card is probably corrupt.
You can try fixing the SD card with disk checking tools present on a PC. If you're unlucky and those tools fail to do the job, you might need to format the entire SD card (losing all files that are on there).
If another device can read the files on your SD card fine, there's something else wrong with your tablet. You could try inserting another SD card. If that SD card can't be recognized either, there's something wrong with the SD card slot on your tablet. If another card works fine there might be a system file or data file causing trouble on your tablet. You could try cleaning up the SD card using another device, removing non-essential files.