1994 Cadillac Deville Very diffcult to steer

I started having problems with my car steering. Turning the wheel all the way around was hard to do, but once I got going down the highway it was fine. The problem kept getting worse so I took it to a mechanic. The mechanic told me that the fluid was ok and the steering pump appeared to look fine. He also told me the problem shouldn’t get much worse; however this statement turned out to be false. It did get worse, to the point I could hardly turn the wheel, it’s like I don't have power steering at all. What do you thing the problem is. Thanks