Try leaving it on charge for about 15 minutes .
or
•Connect the phone to your computer with a USB cable that came in the sales package with your Lumia phone. If the battery has not been used for a long time, to begin charging, you may need to connect the cable, then disconnect and reconnect it.
•Press and hold the Power button for 10 seconds, and then press to turn on your phone again.
•Your PC should detect your phone when it is connected using a USB cable, if this is not happening you can manually switch the phone into specific mode following these steps:
1. Disconnect your phone from the USB cable.
2. Hold down the camera button.
3. Continue holding down the camera button and connect the USB cable between your phone and PC.
4. Continue holding down the camera button until you see the connect-to-computer screen on your Lumia phone display.
5. Release the camera button.
6. The Zune software on the PC should automatically detect your phone now and charging indicator appears on the display.
If i have helped at all a click on the white star below would be nice thanks.
Proud to be using a Nokia Lumia 920