Orange light stays on but nothing come on the sceen

Can you please tell me which light you are talking about? I will assume for now that you are talking about the four on the back of the computer.

There are four small lights on the back of this computer, located right under the video ports. These are called diagnostic lights, and different combinations of green and orange lights represent error messages. Please tell me which order the lights are from left to right.

Depending on this order of green and orange lights, your computer may have many different problems. Consult the page 2 in the user manual for your Dell Dimension 2400 for info on your diagnostic lights and their order's meaning. The manual can be found here: https://dl.dell.com/manuals/all-products...

If your diagnostic lights are all green and the problem still persists, try a different video cable or a different monitor. If the problem still persists, try a different power cable and/or a different power outlet. Unplug all monitors except one, and ensure the right video port on the computer is being used. Unplug any and all printers as well, and disconnect external drives and thumb drives. Make sure your monitor is configured to use the right input, such as HDMI1 or HDMI2. Try with a different monitor if you are not sure.

If the computer is still unresponsive, try unplugging all DVD, CD, or floppy drives. See if the computer boots without them plugged in. More info on how to disconnect these parts can be found in the same manual posted above, on page 10. If the problem persists even more, test and/or replace the power supply.