Referencing this issue (https://bugs.mojang.com/browse/MC-147708)
This isn't a bug. See this forum post for a possible fix (https://www.minecraftforum.net/forums/support/java-edition-support/2690378-failure-to-load-world)
This isn't a bug and will be closed shortly. Restore from a backup or see this article (https://help.minecraft.net/hc/en-us/articles/360034754912-Backing-up-Minecraft-worlds-) to backup worlds in the future.
This is a feature of Minecraft and not a bug.