Some computers might not start at all (they won’t display anything on the monitor even if the lights are up). This is because they are configured to run with all the hardware plugged, even PCI devices like PCI USB, network cards (more than one), old modems etc.
If your PC is displaying on the monitor a message like this: “check cable error”, make sure to have all the hardware plugged in, besides checking the video card that seems to be the number one reason.
If you hear a long beep when the PC starts and also nothing will be displayed on the monitor, check the power source cable, the big white or black rectangle that goes on the motherboard. Remove it and insert it back just to be sure.
Another reason for these issues might be the power source, if it doesn’t deliver enough electric energy, the monitor won’t start at all.
Finally, the motherboard can be damaged and it might need professional repairs.
It is a good idea to always try with hardware parts from friends or from an old computer before buying anything new as most of the time is hard to tell exactly what hardware part is causing problems.