Sorry for the obvious but always good to run down the list as I've made these mistakes enough times to double check.
- Do you have the monitor directly plugged into the back of the video card (not the motherboard)?
- the PCIe power is plugged into the videocard?
- Card is fully seated, clicked in?
- bios setting is set to auto or discrete graphics card
- using the topmost pcie (the x16 one) slot on your motherboard?
- monitor set to the correct output source (i.e. HDMI, DPI etc.).
Those are some troubleshooting tips off the top of my head. If those didn't work, maybe try a power flush, unplug PC from the wall outlet and press the power button on the PC while unplugged several times, wait a couple minutes, plug back power and turn on.