Key broke off in the lock

First, stop

The most expensive thing you can do in the next thirty seconds is keep working at it.

Do not push the broken piece deeper. If it goes past the face of the plug there is nothing to grip and extraction becomes a specialist job.

Do not try to turn the piece with a screwdriver. That bends pins, rounds the plug and can turn a five minute extraction into a drilled lock.

Do not use glue. The advice about superglue on a matchstick circulates constantly and it does not work. Glue does not bond well to a keyway full of graphite and metal filings, and when it fails it fails by gluing the broken piece to the pins. That lock is now a replacement.

If the door is open and you can get in, you have time. Deal with it properly rather than quickly.

If a piece is sticking out

This is the good case and you can often solve it yourself.

Get needle nose pliers, or fine tweezers if the protrusion is small. Grip as much of the exposed key as you can.

Pull straight out along the axis of the keyway. Do not twist, do not lever, do not rock it side to side. The key broke because it was weak, and twisting it snaps it again in a worse place.

If it will not move, spray a dry graphite or PTFE lubricant into the keyway first and try again. Give it a minute to work down into the cylinder.

Do not use oil, WD40 or any wet lubricant. It works briefly, then collects dust and turns to paste inside the cylinder, and it makes any future extraction or rekeying harder. Every locksmith can tell which locks have been sprayed with it.

If nothing is sticking out

A locksmith uses a key extractor, a thin tool with a barb or a spiral that slides down the keyway beside the broken piece, catches on a cut, and draws it out. It usually takes a few minutes and leaves the lock working.

There are do it yourself versions of this. A jigsaw or coping saw blade with fine teeth, slid in teeth downward and hooked into a cut, works sometimes. So do two thin picks used as pincers.

Be honest about the risk. Every failed attempt can push the piece deeper or bend a pin, and the cost of getting it wrong is a drilled lock plus a replacement plus the locksmith you were going to call anyway. If the first careful try does not work, stop.

Expect 75 to 175 dollars for extraction, more at night. If the cylinder is damaged or the piece cannot be recovered, expect the lock to be drilled and replaced, which adds the cost of the new hardware.

Why it broke, and how to stop it happening again

Keys almost never break without warning. Something was wrong first.

The key was worn. A key that has been copied from a copy from a copy is thinner at the cuts than the original and eventually one of those thin points gives. If you have been jiggling a key to make it work, that is the warning.

The lock was binding. A dry, dirty or corroded cylinder needs more force to turn, and force applied to a thin brass key is how it snaps. A lock that has become stiff is asking for service.

The door was misaligned. When a door has dropped or swollen, people lift or pull on the door with the key still in the lock, which loads the key sideways. That is a hinge and alignment problem being solved with a key.

The key was being used as a tool. Opening packages, scraping ice, prying lids. Brass is soft.

What to do once it is out

Do not just cut a new key from the broken halves. The key that broke was already worn, and a copy of it inherits every bit of that wear plus new error.

Ask for a fresh key cut to the lock's code, or decoded from the cylinder. It will work better than the key you have been using, and it will work in every cylinder on the property rather than just the worn one.

Have the cylinder serviced while the locksmith is there. Cleaned out, new springs if they are weak, and a dry lubricant. Ten minutes of work that stops the next break.

Check the door alignment. If the bolt does not extend fully without lifting the door, fix that now.

And if the lock is old, worn and on an exterior door, this is a reasonable moment to replace it with something graded rather than repair it twice.