What is the reason? A simple answer to this question is impossible. This can be a side effect of some medications, e.g., antihistamines. The tongue may be numb as a result of problems in the cervical spine (e.g. osteochondrosis).
Numbness of the tongue can sometimes be anemia, allergic reactions, diabetes. It also occurs in some cardiovascular diseases. In the most severe cases, it can be a symptom of disorders of cerebral circulation (stroke) and a harbinger of the "full" stroke. So better to consult a doctor and get tested, and if necessary to carry out a course of treatment.
In dental practice numbness of the tongue quite often. Typically, this occurs after removal of one or more teeth of the lower jaw. The fact that the nerve of the tongue held close to the tooth roots, and the difficult removal it is easy to damage the dental tool: to squeeze or break. If it was limited by compression, the nerve will recover in a few weeks (on average, a month), but if there was a gap for something more long term.
In the most severe cases, when the sensitivity of the language is not restored even after many months, the long-term physical therapy and medications prescribed by a neurologist.
The causes of numbness of the tongue may be different, so you must first make a correct diagnosis. This should be examined by a dentist, neurologist, endocrinologist. Treatment will be prescribed after a comprehensive examination and definitive diagnosis.