Deathslittlekid

I tried the command, /seed but it said I did not have the permission so I went over to the logs and I found the seed. (On the server, I tried the command. I wanted the seed on the server, and set my own single-player world to that seed.)

I was playing with a few of my friends, then when we all headed off, I got back on, and it was all removed. It was on 1.16.1, completely vanilla, no plugins, all survival. I did try to get the seed over to my singleplayer world, so would that be the cause?