Yesterday evening, my Kryptonite U-Lock was pretty stiff when I unlocked it after work. The key turned fine, but removing the shackle from the crossbar took a few minutes of pulling and jiggling. I eventually was able to separate the two, ride home, and lock the bike up outside. This morning, I couldn't separate the two, even after 10 minutes of finagling. The key turns 180 degrees and presumably the lock is open, but the two parts will not separate. I put some tri flow on the joint, no luck. What is causing this? How can I remove the lock, and prevent this from happening again? For what it's worth, these have probably been the two coldest days of winter so far in my area.
Sounds like you have moisture in there somewhere and its frozen. Try pouring warm (not boiling) water on it from a jug.
When you get it open, take it inside, dry it, and store in a hot water cylinder cupboard to drive out moisture. If you don't have a HWC then 30 minutes in a normal oven on super-low temperature would do. Or even putting the open lock in a sunlit window for a whole day.
Once its dry you need to keep the moisture out - I'd use a lot of grease like White Lithium.
You also want to check the seals and perhaps arrange the lock with the keyhole facing down, and the curved part of the D would be lower. That way rain water won't run into the lock along the sides of the D.