3 most likely possible problem;
1. Cable - check that it is plugged in properly or replace it with verified working cable.
2. Monitor - Verify this by using a working monitor.
3. Graphics card/Motherboard - If the above mentioned doesn't fix the problem then this might be due to the graphics card or motherboard. If you are using a dedicated video/graphics card, remove the cable (i.e. vga) and directly connect it to the motherboard's built in video card before powering "on" the CPU. If you're not using any dedicated video/graphics card, then the problem is your motherboard.
Hope this solves your problem.