Some things to try;
1. Ensure ALL power connectors to the Graphics card are plugged into with the correct connectors, and connected firmly. ( You'll usually hear an audible click or just feel them latch)
2. Reseat the Graphics card. Unscrew the GPU, remove it from it's PCie slot , make sure there's nothing in the way of the GPU on the motherboard, and carefully place the GPU back into the PCie slot.
3. Reseat the Graphics card into a DIFFERENT PCie slot. ( Sometimes PCie slots stop working )
4. Ensure you have an adequate Power supply to feed your Graphics Card.
5. Wait. Sometimes it takes a minute for the Graphics card to send a video signal to your display.
6. Seems pretty obvious, but make sure your display is on the correct...