Is your monitor plugged in? Is it on the correct channel?
You're going to want to take pretty much everything you can out of the system. Take the CPU, CPU cooler, one stick of RAM, put it all in the motherboard. You'll need to use CPU and Mobo power from PSU, then short the PWR and GND prongs on the front panel header. If all is good and the system boots to post, add the other stick of RAM, then GPU.
If the system seems fine, then monitor has to be the issue, so you'll also want to try the monitor with something else, a console or another system, anything. Alternatively, try any other monitor/TV with your system to see if your system displays on the other monitor.