If your card doesn't have an Apple or Mac Edition ROM, the system won't show an image until OS X boots - this is normal. This is not normal on cards with an Apple or Mac Edition ROM. It's possible to flash a ROM to some of these cards, but this will void the warranty if you RMA it with the custom ROM flashed.
If you have verified you have a Mac Edition ROM or Apple ROM card, try using a different monitor. If this works, then you need to replace your display. If it still happens, check if the card requires PCIe power and if it's plugged in. If it does and you don't have the cable, you will need to order this. They generally aren't expensive and are easy to find.
If your card is hooked up and you have verified your card has the correct ROM and has PCIe power, you will need to replace the card.