GPU? Monitor? Connectivity (VGA, DVI, HDMI, DP?)
Have you tried another known working video cable as applicable? Or even another known working monitor or supported TV?
If necessary try another video port. The immediate objective is to get a display and then go from there.
Who built the computer?
For the most part, the starting point is to shutdown, unplug, open the case. (If you did not build the computer then be careful about violating any warranties. If a local builder take the computer back. Or otherwise contact the seller.)
Referencing the motherboard's User Guide/Manual along with all other applicable component installation guides carefully double check that all connections, card seatings, RAM, and jumpers are fully and firmly seated.
Any lights/LEDs on the motherboard? If lit, refer to the troubleshooting section or other applicable sectdion to determine what the light is indicating.
If connections, seatings etc. are correct then again verify that installed components are indeed supported by the motherboard. Most likely you will need to go to the manufacturer's websites to look for more up to date information and details.
Take your time, read carefully. Pay attention to all fine print, warnings, caveats, and so forth.