Screen is black. the power button is blue. i turn off the monitor, then back on, and it flashes the desktop for about a second. then blacks out again. power button is still blue.

One bad lead can cause a computer to continue on a cycle or to shutdown or fail to detect your hard drive

Test all leads that attach to your hard drive including electrical extensions,IDE,SATA

the leads from your ((motherboard to your hard drive)) make sure they have a secure connection and are not faulty or just replace them there probably old and faulty
make sure all leads that are attached to your drives dvd\cd 3 1/2 inch floppy have secure connections and are not faulty even the electic extensions or just replace them they are probably old and faulty a computer needs its connections to have an end so any faulty leads will end up with a computer error

hope this helps