Google and this forum that's itHi-- I'm just curious-- where do you learn these basic facts about a Droid? I have only figured it out by painful trial and error, first thinking I had bricked by Droid by rooting it, then thinking I had a bad phone. If there isn't one, it would be terrific to have a sticky that lists all the most basic, quirky features of the phone.
I've seen the opposite be true, too. When the battery is completely dead and you plug it into AC, it will start to boot, but the battery will die before it gets all the way booted and is able to charge. It'll get stuck in a boot loop without being able to charge further.
In that case, you plug it into USB with it OFF (USB from a computer, not just from a power brick). The white light will come on but the droid won't boot up automatically. Just leave it sitting there for a few hours. It takes a long time to charge, but once it has enough it will boot up on its own