EVERYTHING'S DISAPPEARING!!!!!!!!!!!!

Everything is disappearing in my survival world. This has happened three days in a row. Want an example? I made a gigantic glass roof, which of course, required sand. Today, I checked into my world, and the entire, 156 block roof was gone. Here's why I know it's not because I didn't save it: All the sand that I got for it has also disappeared. SOMEONE HELP ME!!!!!!