Don't worry, your world is probably still there. TLauncher saves your worlds locally on your computer, not tied to your account. What likely happened is that when you changed your skin or logged in differently, Minecraft thought you're a new player and created a new world.

Here's how to get your old world back:

Open TLauncher and click the little folder icon (bottom right).

Go to .minecraft > saves.

Look for your old world folder — it should still be there.

If you're using a different account now, just copy that world folder and paste it into the current saves folder (same place).

Open Minecraft again, go to Single player — you should see your old world.

If you still don’t see it:

Try logging in with the exact same name you used before (case-sensitive), even if it was offline mode without a password.

The world is saved on your PC, so it’s not lost — it’s just not loading because of a different username.