My PC show a black screen after I pressed the bootup button.

Anyone help me please !! When I pressed the start up button for my PC it start up. But the problem is it only show a black screen on my monitor and keyboard and mouse is not responding. I am pretty sure the problem is from my PC (the big box I called it) . So anyone have the solution to this problem? This problem since 1 month ago. And sometimes (very rare )when I started the big box it start up normally like everything is OK.