Reseat all cables and devices.
Try with one RAM chip at a time amd reboot.
Do you get to a DOS screen or see the POST? Can you get into the BIOS setup?
If so set it to boot from CD ROM and boot with the XP CD and get into the Recovery Console and type: FIXBOOT
http://support.microsoft.com/default.aspx?scid=KB;en-us;314058&
I don't turn of my computer usually, but couple of crashed recently made me to. System goes down, but fails to restart. Normally keyboard doesn't light up and all activity is restricted to CD drives blinking and one of them whirring intermittently. Tried to disconnect the cdroms - didn't help. Eventually - after maybe 5 restarts (PC reacts to prolonged OFF-buttoning) or 5-7 minutes waiting it all goes up: cdroms stop blinking and the XP loading screen appears. Couple time I shaken some wires and ribonns internally, but I don't think it was this which made the difference. So, I am at loss. I haven't put any new software and don't even know where to revert with system recovery. Thank you.

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