Jammed Giant brand U-Lock. Unable to insert key
I've had this same issue happen with my Masterlock, and generally remedy it with lots and lots of wiggling. It mostly happens to me in the winter when a bit of moisture gets in there, so I don't know if it's the same issue or not.

My method was to stick the key in as far as it would go, and then repeatedly try and wiggle it back to the most locked position, while moving the key back and for laterally (or whatever the perpendicular direction would be to whichever way it turns.) Just go back and forth a lot - sometimes it works quickly, but sometimes it takes ~15 minutes. Either way, that's always eventually worked for me.