monitor want turn on?

well i have been on here for over three weeks and still dont on why my monitor want turn on,most of every body says it the graphic’s card/on board,so i installed four new video cards,and still my monitor will not turn on,i have tryed other monitors,i reformatted hard drive,that’s it,computer was working befor that,runing slow but it worked,but now it just will not turn the monitor on,i have aother computer that is that same,so i checked the ram,the hard drive,the power box,so some one please help me,why want the monitor turn on,computer turns on but monitor will not?