Most probaly you have been hit by sand.If you were mining, a possible gravel block might have hit you, and because of your lag, therefore stopping you from getting out, killing you slowly.This happens to me sometimes but to cut the answer short, No, you cannot get your stuff back once you die unless you go to the exact location you died before they disapear.And if you burned in lava, the items would have burned already.
I hoped I helped!