First, did you try restarting the tablet (turn it off and back on)? I know it sounds odd to ask, but you would be surprised how many people don't do this.
Now, if that didn't work, you haven't reset the device, and the SD card was set as "Internal" in that device, then it should read it. Have you tried taking the card out and putting it back in? Sometimes they just get a little moody and need to be reinserted.
If that doesn't work, then while the card is in the device, connect the device to a computer via USB cord. Can you access the SD card through the computer? I say this for getting files from it, if it will access it. You can't, however, access apps this way.