OK the spring loaded button behind the sprocket is the pump that automatically pumps when the chain is turning. If nothing broke you need to find a way to hold the "brushes" open so you can put it back in place. The only thing that would mess the pump up would be wear on the parts, a blockage from dirty oil (always clean the area around the fill cap good before removing the cap to fill) or the wrong oil was used (thick summer oil used in cold winter weather for example). Saw dust can often plug the oil passageways too so you need to clean it well whenever you take the cover off.