What is the make and model number of the PC or motherboard if a custom build?
Is the PC booting up OK, you didn’t say?
If not booting up connect a PC system speaker (example only) to the motherboard speaker header pins and check for any beep error codes, (different BIOS makes).
Maybe the PC is not passing POST. Check the motherboard’s user manual for the location of the speaker header pins on the board. Also the BIOS make should be listed as well
If the PC boots OK, does the motherboard have onboard video i.e. CPU with integrated graphics and video output connector (HDMI, vga etc) on motherboard?
If so remove the GPU card and connect the monitor to the onboard video and check that it works OK.