I had a friend with a Brasila that one side hadn't been used in several months... same thing, no water out the group. What I did was remove the screw that crholiday recommended, pulled out the screen and put about a teaspoon of citric acid in the hole. Soak the screen in Citric acid then put the screw back in. Turn the machine on and run the pump. Let the machine sit for a few hours (even overnight) and the run the pump. It took two doses of citric acid and then good as new. You might also want to remove the group screen and soak it in citric acid too while you are waiting.
You can get citric acid at any beer or wine making store, or at most commercial restaurant supply stores.