The grandfather's paradox is based on the assumption that if time travel were to take place and you would kill your own grandfather before you were conceived, your father and by extension yourself would never have been born making it impossible to travel in time in this way. that you could never do it for not having existed.
This paradox is solved with the theory of parallel universes (currently in vogue for theoretical physics), that is, if one travels back in time and changes something of the past, what one would really be doing would "create" a new timeline. independent of the first. So the future of that timeline would be different from the future where you come from. Another thing would be how to return to "your reality" of origin.