Broken Key Extraction
Snapping a key in the lock is actually very common. Both cylinder locks and mortice locks can become victim to a snapped key and both will stop you in your tracks and leave you on the wrong side of the door.
Unfortunately a broken key is usually only possible if the lock itself is faulty causing you to turn the key with to much tension and therefore snapping in your hand. Locks can fail and get stuck due to many reasons so the chances are once the key is out the lock may need to be replaced. Of course its also possible the key had a crack and was just waiting for its chance to snap completely and a new key would work the lock fine, its just a case of trail and error and inspection.
As a professional locksmith I carry specialist key extraction tools which when used correctly can get the broken key bit out without causing any damage to the lock itself. I can then pick the lock allowing you to get indoors while I find out why it happened in the first place and deciding with yourself what the best course of action will be.