Well, when I first started playing Skyrim, I flipped plops the first time this happened to me and loaded a previous save. I didn't realize what happened, and thought my weapon was gone forever.
Now, as I am a little wiser with Skyrim, I've noticed that sometimes your weapon just flies across the room and you have to go find it, instead of it being on the dead Draugr. One time my weapon flew all the way threw the double doors I went through to get to the Draugr. So I always save now before fighting the big guy at the end. If they disarm me, I immediately go look for my weapon on the ground. If I can't find it, I reload the save JUST in case it's not on the Draugr. Of course I only do this if I'm carrying a really expensive and useful weapon.