I had the same happen to me. I left the phone charging overnight
connected to my laptop. I think my laptop went to suspend and
in the morning the phone wouldn't show any signs of life.

I didn't have an external charger so I brought the phone to a local
Nokia care center and they said that if they don't spot the problem
there they have to send it forward for repairing which could take from
3 to 6 weeks.

I'm nevertheless interested in hearing if someone finds out the
actual reason/solution for the problem. So please don't forget to
post if you happen to succeed.