I have a 3 valve style tub shower. The left is the hot water. The right is the cold water. And the middle redirects the water to the shower head.

I have a problem that just occured this morning as everything was fine last night. I can turn on the hot and cold water into the tub fine, but when I try to redirect it, then all of the water flow stops, both into the tub and nothing from the showerhead.

I'm not familiar at all with plumbing but can someone explain to me what the most liklihood of this problem?