It was turned off, but not unplugged. Tested the battery, and it has juice. I have not noticed a red LED anywhere inside that is obvious to me, but I can look again.
After leaving the battery out for several hours and reinstalling, pressed the PMU button (on mine it is not anywhere near where it is indicated on any of the pictures; it is labeled, which I consider a clue), I restarted it once with all peripherals disconnected. Unfortunately, putting it back on my desk & reinstalling various cables, it acted just as before. I've tried disconnecting all, pulling the battery, resetting PMU again, but so far no luck.
I appreciate the help, and am certainly open to any suggestions at this point. It just occured to me, when this happened before, it restarted fine after putting in new battery, but several times I would get a message that the date and time were wrong; when I opened date/time pull down menu, both seemed to be correct. Does this help?
Going to make dinner,and will try again later.
Thank you,Doug