She said, The Philistines are on you, Samson. He awoke out of his sleep, and said, I will go out as at other times, and shake myself free. But he didn't know that Yahweh had departed from him.
The reason Samson lost his strength was only secondarily that he allowed Delilah to cut his hair. The real reason was that "the Lord had departed from him" (Judges 16:20). When God’s Spirit departed from someone under the Old Covenant, the results were disastrous (cf. 1 Samuel 16:14; Psalms 51:11).