Exporting (or as you call it rendering) creates the final video from your project. Exporting a video does not save your project file. You have to use Save or Save As to save progress to your project file.
Most commonly when something says “INVALID” it refers to missing media that was either moved or deleted from your computer. Or it could be as simple as moving the project file to a different drive/folder depending on how your project file was created. You can read more about that here: