Today, I plugged my laptop into J's, as I do, to use it in target disk mode, when lo and behold, I saw that my screen was no longer dark and was flashing the drive image. I went ahead and turned on J's and used it a bit in that mode, then disconnected the two laptops, and restarted mine solo. And ta da. It is working.
My theory was that I had a monitor problem. I guess I have some kind of short circuit thing happening. I may be back; I may not. But it would be great to be able to use my laptop as normal until the shop opens, at which time I'll see what caused the problem while I get them to upgrade it and repair my dvd/cd player.