A little more information about your computer would help. Like, what is your processor, motherboard, RAM, video card (if present), storage drive (HDD/SSD), power supply.
Did you install the CPU yourself? Are you certain it was seated properly? Did you use thermal compound when attaching the CPU cooler?
Is your monitor connected using any type of adapter?
Does the computer issue any beeps when you try to boot it? If so read your motherboard manual (or find it online) and read about the beep codes or POST codes. It should give you an indication of the problem. No beeps?
Shut down the computer, remove the power cord, hold down the power button for a few seconds, touch something big and metal. Remove all RAM, graphics cards, and cables (from the motherboard and also from the drives). Put everything back in, carefully ensuring that each part is connected/seated firmly. Plug the monitor and power cord back in. Try to boot. No boot?
Refer to your motherboard manual and see where you should install just 1 stick of RAM. Follow the preliminary steps given above and remove the other sticks. Try to boot. No boot?
Again prepare to work in your PC and remove the single stick and install on of the remaining sticks (if any). Try to boot. No boot? Repeat this procedure until each DIMM has been tried, one at a time.
Post back your system info and any troubleshooting results.