When the locks on my old car used to seize in the cold, I could heat the key with a lighter, then insert it in the lock and that would be enough to release it. With a u-lock, you may even be able to warm the lock directly with a lighter.
I tried with a lighter, but it was too windy. Also, the U-lock is coated. Strangely the key turns fine, so it is the mechanism inside that is frozen. Heating it would be hard. I guess I could pour hot water on it to get the key out. Oh, the reason the key is stuck is because it won’t turn all the way because the lock isn’t fully closed. Kind of a mess. I have a torch I could bring with me, but I don’t know if I want to have all that going on? Maybe though. Thanks.
Yeah, I might try WD-40. Maybe de-ice and then WD? Something has to work. I guess worse case hot water and keep the lock inside to dry out. Kind of a pain, but I think what happened was that rain and then super cold temps came. Ah well.
About a month ago I had to drag 200 ft. of extension cord and an angle grinder down to the Macy’s garage when my mini ulock seized up for no apparent reason, so, good luck. Otherwise known as “stealing your own bike”. I do not have good history with ulocks. :/