There are many reasons for sudden numbness of legs and inability to walk, generally including physiological causes and pathological causes, different causes have different prognosis. 1. Physiological causes: if numbness and weakness of legs occur after excessive exercise or labor, it is related to lactic acid accumulation; intensive labor or exercise will lead to anaerobic fermentation in the body to produce a large amount of lactic acid, and a large amount of lactic acid accumulates in the muscles, easily leading to local muscle swelling, pain and discomfort, which is a physiological phenomenon that is muscle overload, and can be relieved by resting in general. 2. Pathological causes: mainly include cerebrovascular disease, lumbar spine disease, tumor metastasis and other diseases. (1) Cerebrovascular diseases include cerebral hemorrhage, cerebral infarction and transient ischemic attack. Cerebral hemorrhage patients are given conservative or surgical treatment according to the amount of bleeding, and cerebral infarction or transient ischemic attack are generally given medication. Generally, cerebrovascular diseases can be relieved through treatment and have a good prognosis. (2) Lumbar spine diseases include lumbar disc herniation, etc. Patients with lumbar disc herniation are usually treated with medication or surgery according to the severity of herniation; generally, the prognosis is good after treatment. (3) Tumor metastasis compressing lumbar spinal nerves or intracranial nerves needs to be treated according to the tumor staging and metastatic foci; if the tumor staging is late, the probability of cure is small. When patients have numbness of legs and can not walk, they should consult the hospital in time to clarify the cause of the disease, and carry out treatment under the guidance of doctors.