There are several possible causes for a monitor to display nothing when you try to power on your computer. While some seem obvious, please double-check each step carefully so that you can eliminate possible issues before doing more complicated troubleshooting.
Is the monitor getting power?
Make sure the monitor is connected to power, and that it is powered on. Most monitors have an LED that lights up when the monitor is powered on. Double-check all the power connections and try cycling the power to the monitor, watching for the LED or an on-screen display to indicate that the monitor is getting power.
Example of an LED power indicator on a PC monitor
If the monitor isn't getting power, try a different outlet. If that still doesn't work, consult your monitor manual or contact the manufacturer to get more detailed troubleshooting instructions.
Are the video cables connected correctly?
The next step is to make sure that the display cables are connected between the monitor and the graphics card(GPU) on your computer. The back of your graphics card(GPU) will look something like this:
1. USB Type-C
2. Display Port
4. Display Port
5. Display Port
(See this article for a detailed explanation of the types of connectors and their relative strengths)
Make sure you have the correct cable going from your monitor to the connectors on the back of the GPU. NVIDIA and ATI recommend using a DisplayPort connection, especially for monitors with higher refresh rates(Hz).
Check that the cable is completely inserted into the proper connector on both the GPU and the monitor. Do not force a connector into the GPU or monitor! If it is hard to insert, double-check that you have the correct connectors and that they are in the correct orientation. If you force a connection, you may permanently damage your GPU and/or monitor.
Is the monitor input set correctly?
Many monitors have multiple input connections. When they do, there is a setting to select the correct input, usually using the monitor's built-in "on-screen display" (OSD) menus. Be sure that it is set to either "Automatically detect input" or the correct input type. For example, if you have both an HDMI and a Display Port input on your monitor, and you've connected the GPU and monitor using a Display Port cable, then you should select Display Port as the input. In this case, selecting HDMI will result in no picture being displayed.
Sometimes, your monitor may have multiple inputs of the same type, like HDMI. In this case, they are usually labeled HDMI-1, HDMI-2, etc. Be sure to select the HDMI input port on the OSD that matches the port you connected to the GPU.
For instructions on using your monitor's OSD menu, refer to the monitor's User Guide or contact the manufacturer for additional assistance.
Sample OSD menu showing input selection
(from an ASUS Republic of Gamers monitor)
Still no picture... what's next?
If you've verified that all the previous tests worked, and you're still not seeing an image on the monitor, then you can try replacing the cable that connects the monitor and GPU; you can try a different output port on the GPU (if there is more than one of the same type); or if your monitor supports more than one input type, get a new cable and try a different input type (switch from HDMI to Display Port, for example).
If all these steps are still not solving the problem, try a different monitor (if you have access to one), or connect the monitor to a different PC to see if you can get a picture to appear.
At this point, if you're certain the monitor is working but you can't see a picture when it is connected to the GPU in your BLD gaming PC, please take a look at this article here for some more troubleshooting steps.