ML500 Power steering problems

I have a 2006 ML500. A few months ago, after noticing a severe fluid spill, we had to have to power steering pump and hoses replaced. I drive the car only occasionally around town, and it was back to normal after having this work done, for 2 months. This past week, I was driving around town when the power steering locked up and I had extreme difficulty getting the wheel to turn. Also, I heard that distinctive whining noise. However, when I checked the fluid level, it looked normal. What could be causing the problem with steering if there is a new pump and plenty of fluid?