Why did the steering suddenly stop working on my John Deere 770D grader, and what should I check first?

[Ellen Deuch]

My John Deere 770D grader recently developed a steering problem and now won't steer at all. The machine has been well-maintained and was working fine until the issue appeared. I don't have a service manual or hydraulic schematic yet, so I'm looking for advice on where to start troubleshooting. Could this be related to the steering valve (HMU), a priority valve, a check valve, or a hydraulic pressure issue? If anyone has experience with a 770D steering failure, I'd appreciate any tips on what to check first.