Same thing happened to mine a while ago... one day i had like 15% battery and put it to the charger during the night, next morning it was shut off.. I tried to start it and the only thing that would show up was the 0% battery screen... eventually i tried and got into the bootloader screen, and pluged it into my pc inhopes to reflash the stock rom cuz i thought it was some kind of software related problem, but by plugging it in during bootloader i noticed it didnt even recognise my pc and it didnt charge up. But now all it does is blink the led when plugged in... and when it is plugged into my pc i can hear the sound of something being plugged in and out over and over again..
I already contacted Amazon UK and got a replacement, didnt even contact Motorola cuz they have no stores in Portugal...
I think it must be some software problem or maybe micro-usb related problem... or even the battery being dry..
Ah by the way i tried lots of thing and none worked like holding power while conected, holding power and vol-down..
So if you cant even get in the bootloader screen i dont think you have to worry about warranty, cuz there is no way to access the phone..
Sorry for my bad english...