All my worlds are gone!!!!!

Put the summary of the bug you're having here
All my worlds are hugely corrupted! I have been playing for 2 years almost and it is horrible that everything is gone in a poof. I am very sad because of this and I would greatly appreciate it if you could help fix this.
What I expected to happen was...: I thought that I could just log-on like normal.
Describe what you thought should happen here
I thought I would just play like normal

What actually happened was...:
Everything was corrupted!
Describe what happened here
All my worlds were very very very weird. Superflats would be crowded with random chunks like a normal world, and everything was destroyed.

Steps to Reproduce:
1. I don't know how or why it happened
2. I hope you can fix bugs like this.
3. Please try to get my worlds back, so that the near 2 years aren't wasted.