So I just had this happen to me, though it was a Masterlock brand U lock. The key inserted fully but wouldn’t turn all the way to allow me to open it.
I had just finished a few hours of exhausting bicycle delivery, and it was returning to my bike in the dark after my last delivery that I discovered I couldn’t unlock it. I read a few forums, including this one, and resigned to go home after jimmy-ing the lock for 20 mins.
The next day, I returned with WD-40, a power saw, extension power cable, and a regular saw for good measure - never used any of these tools but they were at my apartment.
By sheer luck, the lock opened immediately, without any need to apply anything. Then I tried turning it again and realized it was stuck again. I don’t know how it opened the first time, but if it didn’t open, every attempt after that would’ve been stuck as well.
As you can see in the pictures, there’s some dumb plastic thing that goes around one side of the U insert (the side farther from the key). Don’t ask me what it does, but apparently the rather soft plastic got damaged, pushed around, and blocked the turning mechanism.
I spent another 30 minutes carving as much of it out as I could with a pocket knife and I hope this never happens again.
Just wanted to make this post in case it helps anyone out there. In this U lock mishap, there were no issues with the key parts. I guess I don’t even have an answer to how it managed to open the next day, but perhaps the next person with this problem to come across this post might find it slightly useful. If that is you, I wish you luck!