I would say the 3D view cannot be loaded correctly so it shows a grey screen. Since the project is still there try saving it and you will need to close out of the program and then open it back and load the save project again. If that does not work you might want to reinstall the program.
You may also want to contact NCH Tech team for assistance, just create a support tckt for them here: https://www.nch.com.au/support/index.html