John,
⌘ Q didn't work (more on this in a minute). I was able to start up in Recovery Mode, did Disk First Aid, everything checked out fine. When I restarted the computer, I saw the Apple logo and status bar; when the status bar got about halfway through the screen blacked out for a moment, then returned, but the status bar didn't move from that halfway point; then the screen went black. The cursor was visible and responded to my mouse, notifications were active and I could activate my screensaver from the upper left corner.
I decided to try restarting in Safe Mode. I was able to get partway in this time, but some interesting things happened on the screen: the screen went blue, with several horizontal blue lines moving in a wave from the bottom of the screen to the top; then the login area appeared. When I entered my password and pressed enter, the horizontal blue lines appeared again, then the screen went black. Again, cursor worked, notifications active, screensaver worked. I tried ⌘ Q and the screen flashed between light gray and black.
So that's where I am. After doing some reading today, I think my Avira antivirus/malware program may be the culprit; how can I get to it to uninstall?
Many thanks!
falguni