Water leaking from sprinkler system (by house) after opening up this spring
That part is probably an anti-siphon valve, required by code for sprinkler systems. These have a one way valve which prevents back flow of water in the sprinkler piping from flowing back into the house in case of a pressure reduction in the house. Turn off the water to the sprinkler system and open up the unit and examine it. 

Simple cleaning of the seal may restore functionality, but you can get rebuild kits for these anti-siphon/back-flow valves. I think it has a replaceable seal which may deteriorate.