Gerber Single Handle Shower Faucet Broken

I'm renting a condo and the Gerber single handle on the shower broke off. Now I have no idea how to put it back on and the water does not turn on. I don't even know where to start on how to fix it. I attached pictures so you can understand the problem better. What do I do????

That pin holds the handle on. You need to go to a plumbing supply house and order a gerber handle.