Hi there!
When you inserted the card in the tablet, did the tablet offer to format the card? Can you pull the card up on the tablet through any sort of 'file explorer', etc? Can you access the information currently on the card through the tablet? It may sound like simple and silly questions I'm asking, but the answers to these questions could help me ...or another person...help you figure out what's amiss.
A few other questions: Does the sd card work in a computer? Or...do you have another tablet or possibly a smartphone you could place the sd card in to see if it functions in other devices as is?