If you see a flashing low battery indicator
If the low battery indicator flashes red and black, the battery has low charge (typically 1%) and is not connected to a power adapter. The computer could also be connected to a lower-power adapter than the one that came with it.
The flashing lasts for approximately eight seconds. Then the computer goes to sleep.
To wake the the computer, connect it to the appropriate power adapter, or wait for the lower-wattage adapter to charge the battery.
If you see a solid (not flashing) battery indicator
If the battery indicator appears on the computer screen for approximately 8 seconds, the battery is nearing the end of charge capacity. After eight seconds, the computer goes to sleep.
To wake the computer, connect it to a power adapter.
If the screen of your computer is black
If the battery is completely out of charge (at 0% capacity), the screen of the computer is black because it's asleep.
To wake the computer, connect it to a power adapter. It make take a few minutes for the battery to get enough charge to start up the computer.