With the limited information given:
Did you test the onboard GPU to ensure your monitor, onboard GPU, cords are all working? After that.. please answer the following questions
What GPU do you have?
If it's a PCI-E card (which I assume it is), did you connect the PCI-E cord into the card/PSU?
When starting the PC, does the fan start spinning then stop?
Did you attempt to reseat the GPU?