I had this happen with a car key. I was advised to put a dab of superglue on the end of the key part remaining, press it firmly onto the broken part inside the lock, count to thirty then gently withdraw. Just to be very sparing with the glue for obvious reasons. Before trying that a garage mechanic gently tapped the lock with a screwdriver handle and the broken part slowly came out. This tapping method worked for me when the threaded part of the jockey wheel clamp broke off inside the A frame. Hope you are as lucky.
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