My son plays Minecraft on his Kindle Fire. His inventory bar went away. How do we get it back?

My son plays Minecraft on his Kindle Fire. His inventory bar went away. How do we get it back?

Added (1). Can you elaborate your answer? I don't plat at all and he's 7… Lol. His sister and him play in a world together. Hers shows Steve's arm, and inventory bar. He somehow lost his?