It is estimated that one million Japanese are missing. They are not missing in the sense that they are nowhere to be found; indeed they are rather easy to locate, usually in their own bedrooms. They are missing in the sense that they shun away from any social contact and for all practical purposes do not form part of Japanese society.
These missing people are called Hikikomori in Japanese, meaning "confined" or "withdrawn". Most of them are young males, in their teens or twenties, usually the eldest son of the family. They decide to withdraw completely from society and confine themselves to their bedrooms for long periods of time, often a few years. They spend their days sleeping, watching TV, reading manga comics and playing on their computer.
Hikikomori is a defence mechanism. The pressures of Japanese society can be overwhelming, especially to young people in the education system. There are entrance exams in every step of the way, starting from pre-school, where the children spend a lot of their time learning how to pass the entrace exam to kindergarten, which in turn prepares them for the entrance exam to primary school, and so on and so forth. Many school students go to cram schools after school hours and on weekends, just to make sure they do not slip behind their peers. The climax of this rat race is the entrance exam to university; many adolescents take a year off after high school to prepare for "exam hell", as it is known, and the more prestigious the university, the harder the exam.
The university years are usually a welcome break. After 18 years of running like mad just to make it to the next step of the ladder, most university students take advantage of their time at school to try and have as much fun as possible. And for a reason; they know that once they graduate they will need to face the job market. Japan's long recession, well over a decade long now, has eliminated the famous "womb to tomb" guarantee of lifetime employment with the big companies. The competition for every available position is ferocious and many find only part-time jobs. This, in turn, makes it hard to move out from home and start a family.
All of this is just too much for many young Japanese, and the result is hikikomori. They deal with it by not dealing with it, by shutting off the world. Initially they may just lock themselves up in their rooms after school or work, but as time goes by and their condition goes untreated (seeking psychological help is still somewhat of a taboo here), they withdraw completely and live in a world of their own. When they eventually emerge, a few years down the road, they are ill-equipped to face the world they have left behind, having lost most of their social skills. In many instances, their way back into society is accompanied with considerable anger and bouts of violence.
These are the missing people. The lost generation of Japan.
For more information, see the Hikokomori entry in Wikipedia.