i just had my whole base on Fishmen Island disappear. I play vanilla and it is, or was my third base. I had my whole group there except one Hive who was scouting. When we came back the base was gone. It is still marked on the map. Anyone an idea what went wrong and/or how to fix this? I imported the quicksave and also the main one which was from yesterday. Nothing.
Unfortunately there really isn't any kind of "fix" for this issue, short of loading an older save. This comes from the game failing to save zone data, the cause of which is currently unknown. You should report the issue on the forums and upload your save.log file found in your Kenshi folder, as that's what the devs need to track down the source of the problem and stop it form happening.
In the meantime, it's highly suggested you keep multiple backups of saves. So that you cannot lose things permanently like that.
This happened to me once when I had a base just north of the ghost town on the Northern Coast. Fortunately I didn't lose anything too important and had another base, but I was about ready to make a post titled "I Have No Base And I Must Scream." I think it has something to do with building an outpost too close to another town.
I haven't had it happen since, but I guess it's because I always have somebody staying behind before setting out. That might be a good remedy if this keeps happening.