Power steering issues

[Mkc55]

So I have a 2004 GMC Serra 1500 , just replaced the power steering pump was running fine. Then a couple of days ago.I drove it home, parked it for the night the temperature dropped to negative 40 and in the morning I went to drive it and I have no steering.

Any ideas to start looking as you can turn the steering wheel but as soon as you put it into gear you cannot turn the wheel. 

[davester]

Is the reservoir still full of fluid?  brakes still feel like they are assisted with the engine running?  what was the time between the pump being replaced and the steering not working?

[Mkc55]

The reservoir is full. The pump was replaced 6 months ago during the summer. Brakes assist.its just turning .