I have the same problem, my world is in the saves folder but does not show up in gameModpack: Direwolf20
Mod And Version: Direwolf20 pack, v1.0.7
Pastebin link to crash log: My client crashed so none, but I attached the crash log from the file.
The bug: I right clicked a tesseract with a tinkers construct pick, and then my game crashed along with my client, and when I tried to log back on to my world, it wasn't in the menu. My other two test worlds are there, and the my world is in my saves, but it doesn't come up on the client.
Can it be repeated? Not in my world...but likely somewhere else.
Known fix: None