Thanks for your answer, MartinRJ. I am a member of the Second Life Beta Group, so perhaps the solution is just to wait for the restarts to be complete.
I have been unable to access the sandboxes for several days, however, so not certain that this will resolve it.
I only joined the group when I was suddenly unable to access the sandboxes a few days ago. Before that, I was not a member, but was able to access.
Now I cannot even find the sandboxes by searching the map.
Very strange.