Hitachi EX150LC Excavator lost control of swing

Gday Earthmovers, I was digging and doing some clearing recently with a hitachiEx150lc and I thought I felt and heard a clunk and suddenly I had no control of the swing. The top half of the excavator swings backwards and forwards on its own. The arm still works and it still drives ok. I checked for broken hoses they were ok. I shut it down and restarted it but still no good. I've parked it on some flat ground and I'm starting my research. Any advice would be greatly appreciated cheers Otis

Hitachi EX150LC Excavator lost control of swing