Same thing happened to me today.
I was trying some ROMs, didn't like it and wanted to restore my old ROM.
In the middle of restoring process, phone died, just like that.
No signs of life - not recognizing charger, JIG doesn't wake it up, absolutely lifeless.... Warranty is not valid anymore (expired 2 months ago).
I will try to leave it without battery till tomorrow, and try to turn it on again.
How can i be sure that motherboard died? I don't want to order new one, just to find out some other part was gone...