Your sure the whole PC isn't stuck in suspend mode?
Light fan etc will all be on but it will appear dead, the simplest fix for it if it is stuck in suspend or hibernate is to clear the bios.
Of course if you have tweaked the bios to the nth degree or have some weird custom settings this might not be a great fix for you.
"If a job's worth doing, no further justification is required."
- Alice: Madness Returns
"It takes just as much work to achieve failure as it does to achieve success."