This probably means that you have started the computer, possibly seen a logo and then hit a black screen on your computer.In this situation I would let Windows fix its own files by using the system file checker (SFC) command.
The first and most obvious reason for a computer appearing to run but you cannot see the video is the monitor. If you see two blue monitor plugs at the back of the computer or even a blue plug and a HDMI port then try using a different plug.
Lines appearing or flashing can sometimes occur, however sometimes nothing happens at all.
You might turn your computer on and have the dreaded black screen.
Some hardware components may not be compatible with the others in your system.
And of course there is always the risk of static electricity damaging your components when you are not an experienced computer repair person.