The inability to open the mouth for no reason may be caused by temporomandibular joint disorder, tumor, trauma, etc., which needs to be analyzed specifically. 1. Temporomandibular joint disorder: patients can’t open their mouths for no reason, which may be a symptom of temporomandibular joint disorder caused by the patients’ recent emotional fluctuation, irregular work and rest, etc. The patients may also be accompanied by temporomandibular joint pain and other symptoms. 2. Tumor: when there is a tumor in the pterygopalatine fossa, or when the tumor in the posterior region of the molar teeth infringes on the internal pterygoid muscle, the patient may have a restricted mouth opening. 3. Trauma: When the trauma leads to the fracture of the condylar process of the joint, a certain degree of mouth opening restriction can occur. There may be other reasons why patients cannot open their mouths for no reason. It is recommended that once patients find themselves in a similar situation, they should consult a doctor in time to improve the relevant examinations and then carry out targeted treatment and therapy.