A few ideas:
If hyd hoses were removed from another implement (like a backhoe) in order to mount something to the 3-point, make sure hyd hoses are reconnected together properly at the back of the tractor.
Make sure hitch down speed valve knob is turned all the way to the Fast position.
Make sure hitch lowering handle is able to bottom out, i.e. the down limit knob isn’t set to keep it raised to a certain operating height.
If it still won’t lower try removing the PTO shaft where it connects to the tractor. If it lowers then the shaft is too long and it’s binding, preventing the hitch from lowering.
Hope this helps.
