An incisor suddenly hurts when you touch it for no reason, it may be a hidden crack, dentin sensitivity, caries and so on. 1. Hidden cracks: tiny cracks on the surface of the teeth, not easy to find, hot and cold stimulation or occlusion to the cracks will appear when you touch the pain. 2. Dentin sensitivity: Exposed dentin, which may be caused by tooth abrasion, gum recession, etc., is particularly sensitive to mechanical stimulation and may cause pain when touched. 3. Caries: Some caries cavities show the shape of small mouth and big bottom, especially fissure caries. It may not be easy to find on the surface, but in fact, it is already deep caries, which may cause pain when you touch it. An incisor suddenly hurt when touched for no reason may also have other pathological factors, it is recommended to go to the hospital in a timely manner, improve the examination to clarify the cause of the disease, under the guidance of the doctor to give targeted treatment or treatment.