After trying and retrying everything I could think of for a few hours, I used System Restore to roll back to a point created two weeks ago. That fixed it. I don't know why it fixed it; it could be a coincidence.
Strange, but it's working now.
All was fine just a week ago. Now, XP (SP3) will not read printer (Canon BJ S500). I turn the printer on, and there is no "connect sound" like there should be. However, the printer is in the folder for printers. A test run "fails." If I restart the PC, the printer is listed but shown to be "offline." I make it "online." Still the same, no response. A mouse connected to the USB port makes the usual "connect sound," so the port seems okay. The printer plugged into a different USB port still does not respond. A different cable still doesn't work.
For some weird reason, XP seems to know that the printer is there, but will not let it print. Why?
If someone can suggest a fix or check, I would really appreciate it.