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Why would a video card stop a PC from starting

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Many years ago someone gave me an ISA modem that would prevent the PC from booting. It was an early PnP card and I discovered it had a fault that was causing it to tie up all of the IRQ lines at once. I ended up "fixing" it by sticking tape over the contacts for all but the IRQ I wanted and then I used it for a few years before it failed to the point that it showed up as a COM port but wouldn't respond.

Investigating on the ATI, I found a problem: the card normally expects a PC's BIOS to set the reference clock speed for the chip, however, with UEFI this doesn't happen because UEFI has a different procedure for initializing the video card.

My card has a TP, and on the DSO I see the reference clock set to the wrong value.

In my hobby project I can force u-boot to fix the reference clock to the right value, anyway, with a modern PC ... I think this is only one of the problems that prevents a legacy card from operating with UEFI.

In some bios setting you have or had  pci  irq's  choices / assignations, it could lock the bios from starting

nowadays you have the : integrated or external  pci video  settings  or even an pci video card boot options  like the integrated one first and the external second  .....  or an auto mode

Best luck would be  reset the bios / clear cmos,  remove all the sloted cards use the internal video if you cpu  has it ... and redo the video settings ? choose external  boot first ???

you dont give specs for the motherbaord, cpu,  video card ......... not much to work with


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