could you please give us as much information as possible about your setup, including the major hardware (graphics card, motherboard, memory, power supply, processor) and any information you might think is relevant.
the usual culprits in this case are incorrectly installed hardware (memory/graphics card/processor not seated properly) and loose or forgotten cables (pci-e power for graphics card, p4 pin on motherboard, EPS12v connector). this is assuming that the monitor never turns on rather than doesn't turn on once booted. basically the cause is generally something that is stopping the computer from passing it's POST (Power On Self Test) therefore the graphics card/adapter isn't turned on.