Rebel:

Did you use an oil based or water based stain on the wood?

How long after staining did you apply the polyurethane?

To your knowledge, was the polyurethane ever "dry to the touch" after it was applied.

If you used oil based stain and oil based poly, you at least have the comfort of knowing that once the mineral spirits finish evaporating, it'll be good. Oil based products cure by reacting with the oxygen in the air. Cold temperatures slow that process, but don't interfere with the mechanism by which that curing happens. So, you can paint a fence in the middle of a Manitoba blizzard with oil based paint (or oil based polyurethane). The fence will be tacky all winter, but when the warm weather comes in spring, it'll dry normally just as though you you'd painted it in mid-summer.