Never had this happen. Started car in 7 degree weather. Went to turn steering wheel and it would not budge. When car warmed up a bit the steering turned. Pump is original with 348,000 miles. Is it possible the pump is getting ready to die?
Check fluid level yet? One day *years* ago, I went to the corner store. All was good. On the way home, I turned the last corner and almost hit a Mercedes. Took some serious effort to turn the wheel. Got home, opened hood, PS fluid *everywhere*. Refilled the reservoir, all was well again. Seriously, around 9-11 years ago.
I suppose it's just as possible the pump is failing as it isn't.
If the fluid is low, and the engine bay around the reservoir is dry, check the boots on your rack and pinion, make sure they have not filled with fluid. My wifes Ranger always leaked via the R&P tie rods, sometimes, the boots sealed enough to keep the fluid inside, sometimes, they leaked.
If the reservoir is full, I'd lean on the pump.
I don't know about our Pontiacs, but, with wifes ford, it wasn't until I replaced R&P and PS pump together that we quit going through them. Sometimes R&P would blow seals, others, PS pump would wine and get notchy / weak even though it was full of fluid.
Best of luck to you. 348K on the car is impressive enough. Sounds like a nice loyal 'bird.
All oil gets thicker when temp fails. At 7 degrees I believe your fluid was too stiff to be useful to the power steering system. But if steering has been good at those low temps, then you may have a problem with the system.