The Philistines laid hold on him, and put out his eyes; and they brought him down to Gaza, and bound him with fetters of brass; and he did grind in the prison-house.
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).