A leader is "in power" while he or she is ruling the country. Afterwards, he or she is "out of power." The terms mean almost exactly the same thing as "in office" and "out of office." ("Power" makes us think of "being in control," "office" makes us think of "having the title like 'President' or 'Premier.'").
When a leader is deposed by an abnormal event like a revolution or a coup, we can use the word "fallen:" he has "fallen out of power."
In the United States, in the year 2000, Barack Obama came into power. In the year 2004, there was another election and the United States decided to keep him in power. Due to U.S. law a President cannot remain in office for more than eight years, and after January of 2016 he will be out of power.