Depending on what you were just doing, there's a lot of things that could cause numbness in the leg. A real common cause of that sensation is if you had just shaved your legs, or if you were sitting in an awkward position.
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You legs go numb, when your blood is either, not reaching your numbed part of the body, or your blood circulation has been cut of for at least 30 seconds.
It might be because there is poor circulation in your leg. That is the number one cause, but I would ask your doctor for the offical answer.