Is there a piece of key sticking out from the lock? In this case, you can simply use a little oil (WD-40 is great for this as well) and apply it to the lock. The best tool for this job is a thin nozzle spray that can manage to properly infiltrate the lack.
After you have greased up the lock you can use a pair of pliers to remove the key. Be gentle and try to rotate the leftover piece a little.
More difficult cases
Things get a little more complicated if the key has broken inside the lock without anything sticking out. You will not be able to reach the key as easily in this case but there are still a few methods that can be tried out. One of these methods is the use of a strong magnet. This works great if the key doesn’t sit too tight.
Of course, you might not have a strong magnet, so let’s find out what else is possible. One surprising but unusual trick is to apply some glue to the key and then stick the broken key back into the lock. All you have to then do is to wait until the glue is dry and you might be able to use your key for one last time.
If this doesn’t work you can use a straightened paper clip, even though it is arguably not the easiest trick to pull off.
If you have no success with these methods, you should read on and learn about the ultimate tools to remove broken keys! They are almost totally foolproof!