2002 GMC Sierra power steering out

Low miles on my truck and never any issues, yet yester drove it 45 miles, parked, and as I was parking it was if the power streering just stopped. Can not turn wheels, not locked, will move a inch or two if you put all your weight into. Fluid, good, no leaks on ground and no warnings steering was having any issues... had to have it towed home. Could the steering box just go out like that? Or what can I check? Is there a fuse in the side panel? Baffled.
Thanks...

jackie