I searched for this issue on the bug tracker and only found a single report on this subject. The report was closed because the player that had reported the bug did not attach a launcher log.

I created a new 1.9.4 Minecraft Superflat world, built structures in it for some time, and exited the game. Upon attempting to load the world again, the screen displayed a single 0% above the center. The game froze and, after some time, no longer responded.

(I have copied all of the text from the launcher log, hopefully nothing in the text file reveals too much private information)

Edit: This bug occurs only when attempting to load a specific world file, and does not seem to apply to other saves.

I have copied the game output log and pasted it here instead of a separate file because it is very short.

[14:12:56] [Server thread/INFO]: Starting integrated minecraft server version 1.9.4
[14:12:56] [Server thread/INFO]: Generating keypair
[14:12:56] [Server thread/INFO]: Preparing start region for level 0
[14:12:57] [Server thread/INFO]: Preparing spawn area: 51%
[14:14:09] [Client thread/INFO]: Stopping!