OK, wiping Data fixed it.
Thanks for your reply. This problem just started this morning with no previous issues, doing the looping part of your problem that I quoted. so it's not a question of what I might have done.
But, for others encountering my part of it, here's what I've done and learned so far today.
As I looked into things elsewhere on the , it seemed to be a software problem. Luckily, I have another Galaxy (Exhibit) that's similar battery and I was able to put the charged battery from it into the SII. The SII booted and worked fine but did not charge. I put the SII battery into the Exhibit (sorta fits). It took a charge.
This confirmed to me that it's not the battery. It must be the phone perhaps USB port or more likely "firmware" since problem started abruptly after draining battery down.
I then wiped data on the SII from CWM/recovery and rebooted.
SII is now charging again. Stock battery is back in and working.
Time will tell if all is well, but here's what I think happened:
1) battery drained down to zero while unplugged
2) plugging in and starting up phone immediately threw something off in the battery stats at a low level...that kept it from charging even when turned off. Or rather threw it into a loop. Don't really understand this stuff, but may have to do with physical things that happen in a battery when the cells drop down way low and then the voltage feedback they give to charging circuit.
3) charging battery in another device (gingerbread, by the way) got the battery out of this, if battery had been a contributing factor,(or just a trigger) it was back over the hump.
4) wiping data fixed the charging, so I conclude it was a charging circuit data issue triggered by an ultra low battery.
Hope I'm over this problem and hope that it helps somebody else. If you run into this yourself with no previous USB or hardware issues, I'd suspect software before hardware. If I had only one battery and no way to charge it externally, I would suggest get one of those simple universal clip-on chargers to bring the battery back up and then go into CWM and wipe data.
UPDATE: It's stopped charging again
. It's no longer looping since I got the battery charged up, but not right either. As soon as I plug it into a USB, it boots itself without powering on.
I guess that this is looking more like a physical USB issue now. Weird that there's been no previous sign of trouble before today.
UPDATE2: I cleaned USB port but no help. Then I bent the USB contacts towards the screen and it is charging now again.
I'm ordering a replacement USB.
I should add that I'm running Shostock Jellybean, older (not current) version with it's associated Jeboo kernel. Don't know if it matters.