The flow of time, why time-quakes don’t exist and who stole the Death Ray blueprints?

Have you ever thought of what will happen to you if some vagrant time traveller goes back in time and in their clumsiness changes things? Worse still, what if their tinkering in our past happened to effect your own lineage. For example what if they changed events which resulted in your grandparents not meeting and therefore never getting married or having children. That would mean that you wouldn’t actually exist anymore!