It is not uncommon to have a bunch of crud accumulate inside the pipes.
On a pressure balance valve like that in your shower, if both sides (hot and cold) are not present when the valve is turned on, that valve will slam to one end. It can stick and prevent flow. Sounds like that's the issue. Sometimes, you can free it up, sometimes you need to replace it. The pros here would need to know the model to tell you the best way to approach the fix.
It's also possible that the showerhead is clogged up with crud. You could try removing it and see if you get flow out the shower arm.