Adam Clarke on Judges 16:17

If I be shaven, then my strength will go from me - The miraculous strength of Samson must not be supposed to reside either in his hair or in his muscles, but in that relation in which he stood to God as a Nazarite, such a person being bound by a solemn vow to walk in a strict conformity to the laws of his Maker. It was a part of the Nazarite's vow to permit no razor to pass on his head; and his long hair was the mark of his Nazirate, and of his vow to God. When Samson permitted his hair to be shorn off, he renounced and broke his Nazir vow; in consequence of which God abandoned him, and therefore we are told, in Jdg 16:20, that the Lord was departed from him.