My uncle's computer got some problems a few days ago. After the prower is on, there is only a black screen. He asked me why.
There are some possible cause:

• Hard disk/CD Rom failure, causing a kind of dead lock of the checking process
• Motherbroad burned down.
• RAM problem
• BIOS problems, maybe there are some kind virus

Finally, the tech guy found that is a RAM problem. After he changed a new set of RAM, all good.