That is an an industrial tractor. Is it an integral loader tractor? You aren't likely to hear from anyone experienced with it on this forum. Someone at yesterdaystractors.com may be familiar with it. So you may get a response if you post there.
Not being familiar with the tractor, I believe it has a hydrostatic type power steering system. If so, your problem is probably a straightforward hydraulic problem.
First off, is the hydraulic reservoir properly filled? It may be the transmission housing on that model. If you need to add much fluid to it, you may have to bleed the system to get it working. Check the Owner's Manual if you have one. If you don't, get one and read it. Do the other hydraulic systems work (3-point hitch and/or loader)? It may have a proportioning or priority valve to direct adequate flow from a single hydraulic pump to the steering system. That valve may be plugged, stuck, or worn to cause the problem.
These are all common areas for problems. How applicable they are and the needed procedures may be model specific. These are details that require familiarity with the model.