Your Terraria server made a new world instead of loading yours

Your world is almost certainly still there. The server loads saves/Worlds/ plus World Name plus .wld, and if that file does not exist it creates a new world with that name. Set World Name in Settings, Game Settings to your world's file name without .wld, and restart.

Get your world back

1. Open the Files tab and go to saves, then Worlds. Find your world's file, for example castle.wld.
2. Open Settings, Game Settings and set World Name to castle. Capitals count.
3. Restart the server. It loads your world.

The new world the server made is a separate file. Delete it later in Files if you do not want it.

Why it happened

• World Name was changed. Changing it never renames a world: it picks or creates one.
• You uploaded a world with a file name different from World Name.
• You uploaded into the wrong folder. Worlds must be in saves/Worlds, not the top folder.
• You switched server type and your World Name had spaces, which TShock and tModLoader do not allow, so it went back to world. Rename the file without spaces and set World Name to match: How to switch your Terraria server to TShock or tModLoader.

Good to know

• Your world is not in saves/Worlds at all? Restore it from a backup: How to back up and restore your Terraria world.
• More about choosing worlds: How to switch between worlds on your Terraria server.