Can our heart stop when we sneeze and if so why?

It could: When you say "stop", remember that our hearts "stop" (pumping) between every beat. That said, when you sneeze, you increase vagal tone, mediated by a valsalva maneuver (forced expiration against a partially closed glottis). This causes the heart rate to slow so the time between beats temporarily increases, ie, the heart spends more time "stopped". Pauses >3 seconds would be unusual.