Just don't panic and keep eye on the topic. By this way, hopefully, the boot loader may "wake up" when you again connect it to a power supply. It has such tendency. I have read it on a couple of places in internet and seems to be working.

Of course first try voyagers' suggestion and lets say leave it for a while to charge (or nor) don't think it matters much, to rest, then by holding the power button (30+ seconds) connect it to the USB cable of the PC.