I over 7000 hrs of play time I have never had a settlement leave, except in one game where I deliberately wanted them to leave so I could set up Raider camps guilt free. It does not just happen and there is more than one notice. I do take care of my settlements and avoid just arriving and leaving as that causes huge problems on Xbox especially in that area with the three settlements so close. The fact that supply line is still there says it has not left. That could be a glitch, I need you to answer the question if you can enter build mode in that settlement. Go to the workbench and see what comes up in your hud first if it has the X transfer option available then again it likely has not left but start with that and then hit "A" to activate build mode.
If you enter build mode it has not left, the next thing would be to figure out what is going on. There is an SKK mod that will diagnose your settlement and help sort this out, the mod I am thinking about might even be able to fix it. Although it is limited in functionality on Xbox.
Do you have a recent save before this settlement got bugged? This could just be a corruption in your save.