From this post: A blank window shows after opening an actor blueprint - UE4 AnswerHub
If you open a blueprint and the view is mostly blank like above use the menu of that blueprint editor window and click on Window->Class Defaults
This will show you a message at the top that says “Note: This is a data only blueprint, so only the default values are shown. It does not have any script or variables. If you want to add some, Open the Full Blueprint Editor”
Click the blue link that reads “Open the Full Blueprint Editor.” You will now be able to use that blueprint again like normal.
Why this is happening I do not know. But this will help you get around this bug.