If one of your people is all of the sudden 'down' ask them if they are all right. It's okay to be concerned about another human being. If performance is being affected meet informally with the employee and ask them why. Try to show genuine concern and not 'gossip' style interest. Avoid the temptation to be the savior and hero.
Most people work through their own problems and issues very well on their own.
Simply expressing concern is often greatly appreciated. Work on your people skills so you can make better observation. Make it clear in your business culture and philosophy that business comes first and is most important. Also clearly relay to your people that everyone lives off the business not just any one individual. When this is understood your people will be more inclined to work out their problems since they know that others in the workplace depend on them.