1.
If you turn back or if someone turns you back when you are going somewhere, you change direction and go towards where you started from.
She turned back towards the crossroads...
They were very nearly forced to turn back...
Police attempted to turn back protesters marching towards the offices of President Ershad.
PHRASAL VERB : V P prep / adv , V P , V P n (not pron) , also V n P
2.
If you cannot turn back , you cannot change your plans and decide not to do something, because the action you have already taken makes it impossible.
The administration has now endorsed the bill and can’t turn back.
PHRASAL VERB : with brd-neg , V P