The many theories are all back to:
1) It's impossible/kills everything/stops time/ bad day
2) Multiple futures with parallel universes based on the outcomes
3) The time machine can only go as far back as its creation and it has not been made yet
4) The time machine travels in a closed loop of information affecting a small minority and localized ("Flatterland"- sequel to "Flatland"- also the paradox by Robert Heinlein in the story "All You Zombies")