The water has stopped flowing from the shower, what to do?

My shower and a bath of water under the control of the taps the same with another aspect of change the environment as a stream of water from the bathtub faucet to shower. Pretty standard commands. It appears, unexpectedly, the transfer of water to either the shower head, when you turn the tap transfer simply stops. The water comes out of the tub, but not out of the shower, with the exception of a few drops. You can not think of what could have caused, but I tested with shower and deferred heads lately. Any idea how to solve this problem? With the shower is turned off and still I have the same problem. Ideas?

Here is a little behind the investor to "touch" in the center. It is very easy to repair. Remove the screw holding the handle and apart. Remove the two screws holding the plate. Remove the plate. There must be a metal bar goes into the wall or a metal edge. Remove the pin or screw connection. Look inside and see your problem. Whether you will be able to remove it and replace it or will have access since the panel behind the tubes (probably in the other room! If there is one). Otherwise, it could be a real problem of having access to these pipes, or behind or not in the shower. Good luck!