This all occurred within a completely vanilla survival world in a multiplayer game. No packs or modifications of any sort.

Upon visiting the home menu while within a Minecraft world, the game may freeze momentarily when reopening the game. Usually the game resumes after a short while but this time we've waited over 20 minutes and no change.

It seemed like the game wasn't in a middle of a save so we decided to relaunch the game completely. The game closed with an error message. When we rejoined the world all we could see was a black screen with the buttons that are visible while sleeping in a bed. Clicking "leave bed" crashed the game, and when we opened the chat, we could see that it was being constantly flooded with respawn set messages.

After waiting awhile, the world selection menu opened behind the chat along with the loading screen box on top of that. We could move the selection across the menu but couldn't click any worlds. After a longer wait, the Minecraft menu with the play and marketplace buttons appeared along with everything else. The game crashed quickly after.

After several attempts we've concluded that the world isn't playable, even copying the world crashes the game.