New answers tagged lock
If you look at the lower picture, you'll see that the missing tab served to hold the plug in its recessed (locked) position. When rotated 90 degrees it lined up with the cut-outs, letting the spring (lost, I presume?) push it out to the unlocked position. It may also have helped retain the plug in the lock. More commonly, that's actually done by a reserved ...
Call a locksmith. It's possible to extract the fragment, but may require disassembly and/or lock picking techniques. Cutting a new key will likewise require taking measurements of the pieces and cutting the new key from those.
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