So 3pt was working fine, you changed hydraulic oil filter and oil, then it was not working ? I would go back over everything I done during that maintenance. Changing oil and filter should not cause this issue. We have to break it down to what the root cause of the malfunction is - which is likely no fluid is flowing to the 3pt hitch. Why would this be? This is why folks are asking if you messed with hoses or anything, or if you are certain the fluid is at the proper level - something is causing no fluid flow to the 3pt hitch. Whatever is preventing the flow occured during your maintenance, so go over everything you did backwards. Double check anything you adjusted. If all you did was pop the drain plugs, drain the oil, pop out the old filter, put in new filter, put drain plugs back in, then refill with oil, then It wouldnt make any sense for this simple operation to break the 3pt. Something else was done, or something is blocking fluid flow to the back end.