If it has an on-board card, then check if it is enabled in the BIOS. if it is, then chances are, it is broken, and you might need to buy an AGP or PCIx one. if it is a dedicated card, however, try to reseat(i.e. remove, and place it back) the card in its slot. make sure it is turned off. then turn it on and check if it detects now. if it still does not, then the card might already be broken.
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If it has an on-board card, then check if it is enabled in the BIOS. if it is, then chances are, it is broken, and you might need to buy an AGP or PCIx one. if it is a dedicated card, however, try to reseat(i.e. remove, and place it back) the card in its slot. make sure it is turned off. then turn it on and check if it detects now. if it still does not, then the card might already be broken.