Causes of knee pain and a numb big toe?

Many: And likely unrelated. The list is exhaustive and you should see an orthopedist who might be able to kill 2 birds with one stone for you.

Can be many things: Unfortunately this is a very general question with hundreds of answers without more detail on your condition. But, combined the two can both occur at the same time from a low lumbar spine disc bulge or herniation causing a "sciatica". But, to show you why this is a tough question, it could be something totally different like gout or lyme disease from a tick bite, so much more info is needed.