Stop. The instinct to dig the fragment out with tweezers, pliers, or a paperclip pushes it deeper and damages the pins. If part of the key is still protruding and the lock is in the unlocked position, you might ease it out with needle-nose pliers — gently, straight out, no twisting.
If the fragment is flush or the lock is locked, call a locksmith. We extract broken keys with purpose-made tools — spiral extractors and broken-key kits — that grip the fragment without touching the pins. Most extractions take minutes and leave the lock fully functional.
After extraction, get to the root cause: keys usually snap because they are worn, bent, or poorly cut — or because the lock itself is stiff and forced the issue. We cut a fresh, accurate key and service the cylinder so it does not happen again.