A bit funny able to post to this slow forum (although it's good this second), but can't search seems strange.
If it didn't turn on accidentally and drain your battery, and the tail-cap is tightened as per the previous message.
Maybe you have an updated version with electronic lockout. What happens when you press the power button? With many Nitecore 2 button systems you press both buttons at once to activate or deactivate the electronic lockout.