A can of WD-40
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How To Use WD-40 To Easily Remove A Stuck Broken Key
A broken key will slide out much easier when sprayed with WD-40; however, this simple hack is dependent on a few factors.
You'll need to call in a locksmith if you can’t access the key with needle nose pliers or a similar tool or if the lock and key refuse to return to their starting position.
If this isn’t the case, grab your WD-40 and bend the supplied application straw to a point where it can spray inside the keyway and into the crevices of the lock itself.
Wait a beat for the spray to start working, then pinch the top of the key with your grabbing tool of choice and twist it until it's properly aligned.
At this point, begin gently working the key out by moving it up and down in a steady motion. Be sure to avoid going side to side as that could cause the key to break further.
Once the key is freed, spray some more WD-40 inside to help prevent future troubles, and be sure to lubricate and clean all your locks every three to six months.