Hitachi Excavator EX150LC has lost swing control?

Gday, I was digging and clearing with my HitachiEX150LC excavator when suddenly I heard a clonk, not that loud and I lost control of the swing. The excavator still drives and the arm still functions but the top half just goes where it wants. There wasn't any hydraulic oil hose breaks. I drove it out of where it was onto a flat spot asap and shut it down. When I restarted it it was still the same, no swing response. Any advice would be greatly appreciated, cheers Otis.

Hitachi Excavator EX150LC has lost swing control?