If your design has become slow to edit or not opening, it may be because of the nested groupings and/or shape generators which has caused this design to be too complex.
You can simplify your scene by creating a "Shape" out of reused objects, such as your shape generators or heavily grouped objects. Once an object is a Shape, its sub groups are flattened and it becomes available as an efficient shape in the Shapes Panel under the "Your Creations" category.
To create a Shape out of a complex or repeating shape, you can select the object, then click "Create Shape". Once the shape is created, you can drag it out to replace the more complex object in your scene.
This will simplify your scene and allow more efficient loading and editing within the design.
If your design will no longer open, you may click "Report an issue" in the footer of Tinkercad's site. Please include the URL of your design in the report.