Dead Plant

I would agree with @hashy u got root rot

Looks like a ph or substrate issues. Possibly just amount ov water as well.

IMO They do that when there over crowded. The buds all fight for the same light and unfortunately some on the edges of scrog nets don’t make it and rot off if there’s to many. It’s that or you bent the branches to far in the net. Just chop those stems at the bottom that are dead and try to clean up bigger fan leaves so they evenly get the right amount of light. If you leave them they’ll take even more away from better buds. Good luck and happy growing.