Two valves are essential on the hydralic unit, one being pressure relief valve, the other one being pressure regulating valve. If the later is not working, make sure there is enough oil in the hydraulic unit, and check if the pressure relief valve is closed tight. 
Sometimes the milling cutter switch is not working by turning the on and off button. What you can do is to check the plug and socket connection. Is it loose? Or it's time to check the electrical part of the milling cutter.
