I think you should see a good neuro-physician as soon as possible. Numbness or tingling sensations are usually an indication of a nerve condition. You may have experienced the pins-and-needle sort of sensations before it turned to numbness. These are called 'paresthesia' and could be caused by nerve entrapment near the spine. Such a condition of the thigh is specifically called the 'meralgia paresthetica' and can be caused by a spinal cord injury or something as minor as pressure caused by lifting something heavy. Such a condition is easily cured by removing the cause of nerve pressure. However!numbness in the thigh can also be an indication of serious nerve conditions such as Multiple Sclerosis, Huntington corea, Pakinson's disease and the like. So please do not take it lightly and consult a physician about it as soon as possible.
