The main option you have if the level is corrupt, is to see if you have an older version of the level in your backup folder that doesn’t have the issue.
- ProjectName\Saved\Backup
If that doesn’t work, you could try migrating your assets, level, and everything over to a new project and see if that fixes it. Most likely if the level is corrupted there, then it will be corrupted in the new project. But it might be worth a shot.
Maybe you could avoid all this by working around the issue though. When you say the box is just hanging in midair after the platform falls; has the physics box fallen asleep? You can check this by getting a reference to it and when the platform falls, wake the box.