Isn't that the grandfather paradox? If you travel in time and cause yourself not to exist then you can't travel back in time to stop your existence because you don't exist before you travelled back in time.
If I go into a time machine and assassinate myself ten years in the past then how do I travel back in time? I died ten years ago so I can't assassinate myself because I'm already dead from the assassination ten years ago. If I did go back in time and assassinate myself then what would happen? Would I stop the event from happening by being dead?
I don't know.
