My 2005 model mercer monitor switch on light just frickers but the screen remains off

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

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

the leads from your "((MOTHERBOARD TO THE HARD DRIVE))" make sure they have a secure connection and are not faulty or just replace them they could be 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 electical extensions or just replace them they could be faulty a computer needs its connections to continue its cycle and have an end so any faulty leads will end up with a computer error hope this helps
if this fails to fix the problem
or it might be your graphics card make sure its securely seated and there is no dust built up
on and around the graphics card hope this helps