Large sand / sandstone structure suddely appears

Playing. Monitor broke. Turned off PC. Turned PC back on. Logged in. Suddenly, two large sandstone, sand and stone structure had appeared behind the desert temple I had been renovating. Also, the rear of the temple was missing (whole back wall) and a large lake had appeared further out from the back of my new temple home. I'm not playing on a open server and my kids are too young to have done it.