Does look like the sand has been moved.
Do a 50% water change with water as close to temp, Ph and Sg to your tank as possible. Get the skimmer running. Get the carbon back in the filter. Cover the tank with black plastic for 3 days and leave the lights off also. Keep the skimmer cup clean, the filter material changed out. At the end of 3 days take the black plastic off and do another 50% water change.
This is just my opinion.