I've lost an old machine or two to legacy CRTs that "snapped". Are there any devices that can be hooked between a computer and a monitor to keep a computer safe in the event that a legacy monitor blows? Something with a fuse in it or something like that?
Hmm... over the decades I've had many monitor failures between personal systems and a multi-location company network I manage.
Having one take out a computer??? Never. It'd be a pretty rasty, grotty cheap monitor that'd pass that kind of voltage down the video cable. For the results you describe, your problem's likely a failed ground circuit, which is a safety issue. I'd be more worried about my bodily safety than any fiddling old computer. Make sure the ground wire in the computer & monitor power cables have continuity and that the wall socket ground is functional before worrying about anything else.
No fuse is going to take care of the problem, you're talking about a circuit failure that's somehow, due to bad ground most likely, dumped high voltage into the video circuitry and caused the monitor to use the video cable as its ground via the video card. The VGA signal voltage is 1V or less and it would take some sort of voltage clamping circuitry array that I've never heard of being manufactured to protect your HD-15 connector's pins from passing that voltage into the video card.