Well hardware can matter but more important "how far are you getting ?" Not quite sure what "won't start up again" means.
Does the MoBo logo appear ? , can you get into BIOS ?, does windows start to load ?
Thanks for your reply Jack,
As I said in the subject line, the machine won't power on. If I supply power to the PSU, the green LED on the motherboard lights up to show that it's getting power, but when I press the front power switch, nothing else happens. The cpu and system fans don't fire up and there's no output from the video card.