Losing Tons of Items

My friend started up my server today after it crashed (even though it wasnt online), and randomly, the server reset its items from an earlier world backup, but the actual builds in our world were not lost. Now, I am really confused about what happened, and myself and many friends have lost countless amounts of items that we spent many hours to obtain in our vanilla survival world. Has this happened to anyone else, and is there any way to prevent this from happening again, or some way to recover our progress?

How does the cracked mode change that though?

And now for some reason, my nether portals don't teleport me or my friends into the nether, even though they make the wooshing noise, as if we did get teleported to the other side.