2002 GMC Envoy power steering failure

Check fluid level in PS pump resevour, if OK, then check for leaks in pressure and return lines.
Generally the PS pump will make a whining noise if low.
If no leaks and fluid level is good, I would check outer tie rods. Raise car put on stands and see if wheel is hard to turn, if so, you may be looking at a rack problem, tie rods, ball joints, OR booster vacuum leak, leaking diaphram. To check diaphram, hook up vacuum gauge pump and see if it holds vacuum.