چرا کامپیوتر صفحه نمایش نمیده computer not display the screen?

@sultanatai that will depend on how you have your computer display connected. Of course, you made sure that your monitor is working. Next you want to tell us if you are using the integrated video on the motherboard or if you are using a dedicated video card.

If you do use a video card, switch your monitor to the integrated graphics card and see if that works. If so, it is most likely a failed graphics card.

If you are not using a dedicated card then the issue could be with the GPU on your motherboard. Now you want to try a dedicated graphics card to see if that works.

Use the Dell Optiplex 790 Owner's Manual to work on your computer. It is in English but hopefully it will still work for you.