A Suggest that the individual seek counseling, especially if your organization offers an (EAP) or related resource.
Just don’t press too hard. Generally, it’s best to focus on the employee’s work. Hold the employee accountable for her duties in a fair, measurable way. This employee sounds like she’ll do best in an isolated setting where she can work with few distractions.
Above all, don’t blow her moodiness out of proportion. If she’s downcast, let it go. Don’t let her glum demeanor bother you, as long as she gets her work done and doesn’t pick fights or make the whole office miserable. And resist the well-intentioned urge to ask, “Are you OK?” She might perceive it as nosy.