As I was driving today after work I came to a stop at light to make a turn and randomly the steering wheel locked on me. I turned the car off and started it again and suddenly the problem went away. I checked my power steering fluid once I got home and the fluid level was perfect. I want to know what would cause this. I have a 2007 Jaguar S-Type R V8 4.2L. (Side note the weather was -30C today could that affect power steering?)

 

Don't know but I would change and flush the fluid right now. Do you know about the additive for squeaking steering?
You did not mention any strange noises?

I use a turkey baster and suck as much out as I can that way. Do that a couple of times and you will have changed most of the fluid out.
The screen in the top of the reservoir has 2 clips you can squeeze to remove the screen so you can get to the bottom of the reservoir when sucking out the old fluid.
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Please define "steering wheel locked". If you lose the power assist for any reason, the steering wheel will require much greater effort to turn, but it can still be done. The steering wheel might feel as if it was locked, but it would be a mushy stop if only applying normal force. As long as you put some muscle into it, you'd generally have enough steering control to navigate to the crash site. On the other hand, if the mechanical anti-theft lock (i.e. when you park and remove the key) had somehow engaged, the feel is totally rigid, no mush at all. No steering action would be possible, even if you were Hercules.

It's not too far-fetched that the cold could have affected the serpentine belt. If the belt was not flexing (and gripping) properly in that booger-freezing cold, it may have been gripping only partially at higher RPM, but this was just enough to drive the pump at less than full speed and still provide some power assist. When you slowed for the stop light, the belt wasn't spinning the pump fast enough any more. I'd start by checking the belt tension. Here's an excerpt from a Jaguar training manual showing how to check the belt tension on an early V6. I believe all S-Type model years and engines are similar:




If the problem returns, and you confirm the wheel isn't mechanically locked, I'd be willing to gamble your time and money on a new serpentine belt. Make sure to get a good brand such as Gates. Stay away from discount offerings.