My tub /shower has one faucet handle for hot & cold water . Turn left hot , right cold .?

Last night my son was taking a bath and when he went to turn water off he somehow knocked the faucet handle out of place.

I turn the faucet and no water comes out. There is a little bit dribbling out of faucet. Somehow he moved it ( faucet handle) off whatever it holds onto (pipe I imagine) so can not get any water to come out. I tried wiggling it back and forth nothing.

PLEASE HOW CAN I GET THE FAUCET HANDLE FIXED SO WATER HANDLE WORKS. I turn handle and NO water comes out. I need to fix if ASAP . I have no $$$$ for z plumber. If all he did wax knock faucet into it on wall hoping not to hard to fix . CAN I JUST UNSCREW THE CIRCLE THINGIE ON THE FAUCET AND MOVE IT. ?? HELP PLZ