Note: Always make backups whenever you are modifying any game files!
Minecraft occasionally encounters an issue that can corrupt the save data of a world save. When this happens, the chunks in the game can be reset to how they were when the world was generated, or they can disappear completely. Many factors can cause corruption and, in some cases, can prevent the world from being opened at all. However, not all hope is lost if a world save is corrupted.
The process to fix a corrupt world can be a bit challenging and is not guaranteed to fix the world every time. Making frequent backups of your world will always be the best and safest way to make sure you can recover from a corrupt save file. Always make a backup before you attempt this process even if your world has already been corrupted.
Because this process involves transferring and modifying files on your system, Minecraft support is not able to assist with this issue. However, a guide is available on the Minecraft Wiki to help walk you through the steps. Minecraft support cannot guarantee that this process will work, so please use it at your own risk. To continue to the guide, visit Tutorials/Recover corrupted saved world data on the Minecraft Wiki.