|If it normally gives a single beep when powering up and now it doesn't, that means it's dead and the motherboard is not working.|
All that is needed to give that beep is a working power supply and motherboard. Of course a defective device plugged into the motherboard can kill it.
A Google search for "Troubleshooting a PC" will turn up some help, but if you have a compatible power supply, swapping it might be a good first step. Or you could just try removing all power and reseating the motherboard power connector.
For what it's worth to XP users: When you click on the 'Command Prompt' you are just causing the 'Command prompt' to be displayed. This prompt gives you access to the 'NT Virtual DOS Manager'.