Suddenly not being able to see things with blackness in front of your eyes may be due to central retinal artery spasm, upright hypotension, and transient hypoglycemia. Central retinal artery spasm will lead to temporary ischemia of the retina, and the retinal tissue consumes a lot of oxygen and nutrients, once ischemia will cause impaired function, resulting in sudden vision loss. Upright hypotension is caused by the imperfect regulation of blood pressure by the vegetative nervous system. When the patient stands up suddenly from a sitting or lying position, the blood pressure cannot rise in time and causes temporary insufficient blood supply to the brain, resulting in darkness in front of the eyes. Transient hypoglycemia is caused by the lack of sugar and other nutrients supply to the eyes and brain, resulting in temporary functional impairment, resulting in darkness before the eyes, as well as sweating, rapid heart rate and other manifestations. The patient should go to the ophthalmology department of the hospital in time, and the doctor will diagnose the specific cause through detailed examination, and then take targeted treatment to eliminate the symptoms.