You should charge the iPad for a while using the power adapter plugged into a wall socket. Your computer's USB port is not supplying enough power to recharge the iPad battery efficiently - even though it mat actually be trickle charging the device.
After you have charged the battery for a while, try restoring using recovery mode. It may take a few attempts in order for this to work, so don't quit if it does not work the first time that you try it.
Read about recovery mode here.