Device on, but keys on Casio fx-20 calculator not responding

The batteries are good, the device is on (the 0 is lit), but my Casio fx-20 scientific calculator is not working. It’s not responding when I press any of the keys/buttons. It was working earlier today, and has not been dropped or anything. It’s been gently used over many years. Is there a way to open it up and maybe clean something inside? I see only one screw in the battery compartment, but I’m not sure this opens it up.