If updating the video driver does not help (the usual advice), then it could be a known issue caused by Windows updates...
Look toward the end of the discussion, starting with post 11. Before that, you get advice on workarounds.
Thanks for the link, Gary. Making the secondary monitor the main monitor seems for me to be the best workaround. It's pretty pathetic that this needed to be done but I guess that's what I'll do until a real fix comes into play.