Hi Warren, is there plenty of oil in transmission?
I am assuming since you changed filter and fluid that the hydraulic pump has lost prime. The other possibility is the pressure plate splines have stripped and PTO shaft is no longer turning to drive hydraulic pump. If you have an air compressor with a nozzle tip, you can try removing the hydraulic dip stick and applying some air pressure in the dipstick hole while running engine about 1500 RPM and working steering wheel or hydraulic loader valves. If that doesn't work, place a clean bucket under MCV valve where hydraulic filter is located. On bottom left of MCV should be the high pressure hydraulic line to the steering system, loosen the nut on the line a couple turns with engine running and that should let air out of the system and hydraulic pump to prime.