Saturday night we played mini games on the realm. Sunday night we logged in to our current world (World 3) to discover that the world seemed to have reset itself. 

1.) All our buildings were gone (from all bases spanning 10,000 blocks) except for two walls of the most recently built building (looked like maybe the corner of a chunk?). 

2.) The land went back to the way it was before any terraforming was done. One base that was built in the forest not only disappeared but all the trees grew back as well. Creepers holes were filled, etc.

3.) The nether reset and all our tunnels were gone. 

4.) Our achievements reset.

5.) We all spawned where we had last logged off (not all at spawn like if we reset the world), kept whatever was in our inventory, the contents of our enderchests, and our xp. 

I was able to go back to a previous world save but I am hoping to avoid having this happen in the future. It was just an interim world while we wait for the update so we aren't terribly attached but it was still alarming. Lol