Sherlock dies. Filler picks the setting...or timeline (maybe Sherlock did die in the fall, maybe he did overdose on the plane, or killed himself in euro's cell.) Point is...he dies. John completely loses it and blacks out.

Only to wake up when it all started....The day he met Sherlock. John retains all of his memories and sets out to fix things.

tl;dr: john's love for Sherlock is so strong it tears apart a hole in the space-time continuum.