Hey I had this prob when I repleased my PS hose (high press. side, pump to box) I got steering locked as if I don't have a power steering. Then I stoped the car, Checked PS fluid (ATF in my case), it was ok, since I had put the fresh one after PS hose replacement. Then I stoped the engine and restarted it. It was locked as it was before. Lock is not the right word here, I would say hard , like without power steering like older cars. I push with both my hands on both the directions and then I heard bubbling sound and my PS gets to normal and easy. Then I checked PS fluid rev, I had bubbles on top of the fluid. So I assumed air pocket in PS system coused this problem, since I had to drain and refill for PS hose change. Since then I never have any problem.
I would say try the check the fluid level, if it ok then you can try flush and refill before going to any expansive repair.
Good Luck
Akhil