Be honest with them, and MUCH more important: Be kind, when discussing this. Something along the lines of 'Hey, I realized that something seems to be off since a while, is there anything I can help or change, because I really want you to be happy and productive' works best for me.
Pretty much everyone wants to be recognized, happy and productive in their lives, and this usually extends in their jobs, so most likely, that person needs help, to recognize or solve the situation.
Sometimes, they just dread for whatever reason the task they have ahead of them, and need someone to talk to. Sometimes it is some time to solve a challenge in their live outside of work - then it might be your job to shield that person to some extent, while also making sure this does not take a toll on your remaining team. Sometimes it is a social problem in your team. Sometimes the assignments for that person dont match what they can and want to do. And of course, sometimes the team, the job, or the company is just no fit, and it is perfectly fine to help there as well.
But again, above all: When talking about this, when trying to help, keep in mind that you are the manager, which always implies a power imbalance, no matter what. Such can easily be perceived as a threat instead of help, and if it is perceived like that, you will make the situation just worse.
So be kind, listen, try to coach and help, even if your inner self is annoyed with this. It rarely helps to play the testosterone driven top-down approach, unless, of course, you tried the other ways before with no success.