I have numbness, tingling, and burning in both legs. Sometimes my knees buckle when I walk. When I lie down the legs go completely numb. What is causing this?
What you're describing is called peripheral neuropathy. Unfortunately simply stating that you have peripheral neuropathy doesn't bring us all that closer to determining what's causing it since many things can cause these rather disturbing symptoms. (At one point I had significant numbness in my extremities; my problem was high levels of mercury from eating too much fish!). Peripheral neuropathy refers to problems with your nerves.
This is definitely something you want to get checked out because while the cause can be rather innocuous as in my case, peripheral neuropathy can also be a sign of a serious problem such as diabetes which you'll want to get under control as soon as possible. See your physician as soon as you can. Good luck!
For more on peripheral neuropathy:
Nerve Pain at a Glance
Neuralgia – An Overview
What Can Be Done for Nerve Pain?