Jen,
Thanks for your response. I tried all of these, and was resigned to it being a hardware issue. Because the computer was still under extended warranty (with about 4 days to spare), I took it to BestBuy for repair (which handles repairs of Vaios for Sony now that Sony is out of the computer business). One of the geek squad techs in the store was playing around with it, and was able to fix the problem by reinstalling the Intel HD Graphics Family/Intel HD Graphics 4400 Driver and updates (see https://esupport.sony.com/US/p/swu-download.pl?mdl=SVP132190X&upd_id=8758&os_group_id=24). Yea!