This means that parts of your lungs have collapsed. An atelectasis can refer to a whole section (or "lobe") of your lung collapsing or it may just refer to a few alveoli collapsing. An atelectasis can be caused by a rib fracture, a bronchial plug of phlegm, of by very little air getting into the lungs because you are not breathing well. You will probably need chest physiotherapy to help you get out the phlegm and expand the collapsed part of your lung.