usb will still 'flash' unless the power switch is off, sometimes a charge will remain in the pc, enough to make the small led's light up for a while.
i take it there is absolutely no action at all when you connect power? (as in, fans turning etc)
if this is the case, i would aim towards power supply rather than mobo/cpu failure.
this might sound wierd, but i had a tower that would refuse to start every now and then, i found that by removing the power lead from the tower and pushing it back in, sometimes several times, then eventually it would start.
did you close the pc down correctly before you switched it off....might be worth taking a look at the fuse in the power supply, usually screwed in at the back of the supply.
some towers have an on/off switch at the back of the supply, this can be quite easy knocked into the off position.