yingwu, it's good that you didn't panic, but just calmly put your DR on charge. A charge can solve so many problems, not just with DRs, but all e-Ink devices
As you'd know, the battery indicator icon isn't fully reliable for the first few charges.
There's advice for managing battery charging at
http://www.irextechnologies.com/batteries
While this document is written for iLiads, I expect it's also applicable for DRs (just ignore the parts about stylus, wifi, usb stick)...