Are you able to create a new save in the world from the world select menu? Is the world showing up in your documents folder, under The Sims 3 --> InstalledWorlds? This will at least let you know whether the world is actually installed.
If it is, does the world name have any characters besides standard letters and numbers, maybe a . or something similar? I believe there is a bug where saves created in worlds with non-standard characters in their name can't be re-opened.
If it is not showing as installed in the menu or the folder, you could try creating a new The Sims 3 documents folder (be sure to keep your current one somewhere safe) and installing it to that one to test if it disappears again.
