Thanks for your suggestions. I will look into these. This is a computer I'm working on as part of my job and I've been here a short time, and don't really know what they do and don't do, etc.
I did the "safe" boot up, held down the S, after unplugging all the peripherals, and it worked! Then reconnected them after boot up.
That still doesn't help me know why it really happened, so I'm a little scared to shut down now.
I didn't have an internet connection so I rebooted the computer -- I wish I'd just rebooted the modem now.
When it "rebooted" it didn't boot up but a black screen with a lot of white type saying the start up of various things failed -- it looked like a PC script type thing. Then that type...