This "E:/" part of the path is what troubles me. Ususally the main hard drive is named C:/. It looks like you saved the file to an external hard drive or usb drive. Are you sure that this very hard drive is connected to your PC, and in the same usb port in particular?
Also, are you sure you haven't moved any crucial parts of your game to another directory?
Last but not least, did you break compatibility by opening and saving your project in another version of Godot? (e.g. Godot 3.0 and Godot 3.1 are not fully compatible).