Droopy eyelids (eyelid ptosis) occurs when the edge of the upper eyelid that contains the lashes falls too low. When the edge of the eyelid falls and covers part of the pupil, it blocks the upper part of your vision. In severe cases, it is necessary to tilt one's head back or lift the eyelid with a finger in order to see out from under the drooping lid.
A drooping eyelid can occur as a congenital defect in which the muscle that lifts the eyelid fails to develop properly. Surgery to repair such a congenital defect involves either shortening the weak muscle or, in severe cases, using a "sling" operation to elevate the eyelid.
In most cases, a drooping upper eyelid results from aging of previously normal structures. Typically, the tendon that attaches the "lifting" muscle to the eyelid stretches and the eyelid falls too low.
Since the muscle that lifts the eyelid has normal strength, surgical correction of a drooping upper eyelid that was once normal involves repairing the stretched tendon. It is not uncommon for one to develop a droopy upper eyelid following cataract surgery. The cataract surgery is apparently the "last straw" that causes a weak tendon to finally give way.