Ultimaker Cura has an option to save the current display state of the computer and use that information to open Cura full screen in the correct position every time. Connecting and disconnecting an external monitor in between reboots of Ultimaker Cura, might cause the application to be positioned incorrectly, and therefore hidden.
To prevent this issue from happening, please disable the option in the preferences menu:
- Navigate to the Ultimaker Cura 'preferences menu'
- In the 'general' tab, disable the option 'restore window position on start'