For those you without Macs, the restart is a known fix for many problems, only this time, instead of the little apple, I got the little circle with the line through it, which is bad.
Got a prompt to run First Aid, which I did. Everything is fine, First Aid said, we're all fine here, how are you?
Computer is only eight months old, so I had Apple Care™, but but I still had internet connectivity, so I managed to download and re-install the OS, which in my case is one back, Mountain Lion. (I am waiting until the first round of fixes are done for the next OS, i.e., Mavericks. There are bugs, and installing anything that is version xx.0 is always iffy. Having read the comments about problems, I can wait, it's not going to give me stuff worth the risks.)
So after 2-1/2 hours, the download and restart happened, and everything is working now.
How did I trash my OS-X? God only knows, but after updating it, it seems to be okay, so … there you go. Another pothole in the information highway ...