simply a crystal that when struck generates an electric charge... that charge runs down a wire to a gap... the gap allows the charge to jump, making a spark.

That little button you push is simply a mechanical device that snaps to strike the crystal.

Here are the basics on wiki: https://en.wikipedia.org/wiki/Piezo_ignition

If it stops working, replace it, or at least clean and adjust the gap. On the other hand, if it no longer "snaps," it is broken.