I turn on the computer and the only thing that starts is the fan. Nothing shows on the screen. I push the button on the pc to turn it off but it does not respond. it keeps stayin on. the only way to turn it off is by unplugging it.

Is this machine an AT or ATX? (It sounds as if it's ATX as the power button turns it ON.) On these machines, to power it off, you have to hold the power button in for about five seconds.

Unplug the system and remove the cover. Check all power connections from the power supply to the motherboard and devices. (Hard disks, CDROM's, CPU fan, case fans if any, etc.) If all connections are secure, it could be either the power supply or motherboard gone bad. Without sensitive equipment to check on things (power supply voltages/amperage, etc) a shop may be the best bet to determine what's wrong. If you can swap out the power supply with another known good one this will help narrow things down.

Another thing to try is to disconnect all devices except one memory stick, and the video card. If it powers up, reconnect one thing at a time to find/eliminate faulty devices as the cause.