Someone who's bedridden is so sick or elderly that they can't get out of bed. Your friend might have such a bad case of the flu that she's bedridden for several days and can't go to work or school.
Most people who are bedridden are terribly ill and confined to their bed — or a hospital bed — until they recover. Very old people might also be bedridden because of weakness or pain. The word comes from the Old English bæddrædæn, "bedridden man," from the roots bedd, "bed," and rida, "rider."