You died and your items decayed, check your event log (esc - event log). It is always good practice to log out naked with everything stored in a chest because your body remains in the world and you can die. Alternatively, you can disable “bodies remain in the world” on your private server (if you are the admin).
If it is your own private server, ticking the "character doesn’t say in world while logged out’ option in server set up, will stop this from happening.
If you opt to remove the bodies, it will apply to all players on this server. As soon as they log out, their bodies will disappear. If it is a pvp server, it might be problematic because of “body vaulting”.