Collapse of the lung is due to the mass. the mass obstructs the airway so that the part beyond the obstruction collapses.
Causes could be various. It could be an ordinary tumor
like harmartoma which could be left alone or it could be carcinoma especially if she was a long time smoker, which is serious cancer and might need surgery. It could be lung abscess
if she had low grade fever
and weight loss
. Hope for the best and hope it's benign.