This is a shot from the hip, but I'd be willing to bet you've got the G3 video issue, and that the problem doesn't have anything to do with ethernet. Strange lines across the screen are a giveaway of the video issue, especially for the 700-900MHZ models. It's possible that plugging in the ethernet cable bumped the logic board in a way which aggravated a dormant case of this problem.
Gwyn, when you state you "can't get the computer to start", do you mean it's completely dead, or do you hear power-on noises but get a black screen? Typically with the video issue, it will power on, but the screen will be black (if it doesn't come on and show lines and strange artifacts). Here's a test: While it is off, clamp the computer with your left hand just to the left of the trackpad, applying firm pressure to the top and underside. Power it on while you are applying pressure. Don't be afraid to apply some significant pressure. If the screen lights up, there is a 99% chance you have the video issue. If you DO have the video issue, then you're basically ready for a new machine, because there is no real cost-effective and permanent fix.