Hey there,
This symbol generally appears when you are connected with a charger not suited specifically to your blackberry. the connector may fit the socket on the phone but does not work at the level the blackberry requires to charge. Try another cord made for the blackberry.
Also remove the battery from the device and ensure all the connection points are dust free. I don't think this relates to your problem, but a good idea to do from time to time anyway.
Good luck.