My Sims 3 Pets Game Won't Load - It Says It Can't Find The Disc When It's Already In

I just bought the Sims 3 Pets EP. (It's a disc, I didn't buy it online.)
I tried installing it, and everything went fine. However, whenever I load my game and go to select a world, I get a popup notification saying that I need to put the disc in in order to play. But the disc is already in. I keep clicking retry and yet it still doesn't work. I'm not sure what to do. I can't play Sims until this problem is resolved.
Please help!