Scratch occasionally crashes. Sometimes, the .sb3 file header gets corrupted, but the internal ZIP data remains intact. By using a converter, you can repack the surviving assets into a fresh SB3 shell.
This is the standard way to "convert" a zip to an sb3. Zip To Sb3 Converter
You don't necessarily need a dedicated website or software to do this; you can do it directly on your computer: Prepare your files : Ensure your project.json Scratch occasionally crashes
Usage (quick)
The project.json must be at the root, never compress folders recursively, and always validate your JSON syntax. With the right converter, turning a messy folder of assets into a playable Scratch game takes less than five seconds. Scratch occasionally crashes. Sometimes