Yeah, last night something went wrong with the server.
For me for example all bases I had access to (friend status) disappeared from the map, except the one I am currently member of. My tent is still there, although it is not shared.
I am guessing your friend was the one who made the tent so he can still see it, try having him lead you to the tent again to see if it gets you it back on the map, it might work.
With bases it would almost certainly work, which is what I will be doing this evening...
I talked with a GM about this, and even if the bug which caused this corruption was fixed, there probably won't be rollback, so we should fix it ourselves, hopefully it would be possible in the shared tent case too...
