Well, usually letting it render overnight solves the problem.
Last night was weird though. I left it at about 30% done, to chew on overnight, and in the morning, Studio was open and running, but there was no scene in the viewport, and nothing in the scene list, and the render window was gone.
I've no idea what happened. A power outage would have lost the render, but it wouldn't have re-opened Studio. The best I can guess is that it was something from the OS (which had started nagging me to install a system update, only I wanted to wait until the render had finished). That's only a guess, though. The "update me" notice was still showing, and had certainly not been done in my absence.