Leave the device to charge 2-3 hours, and then, still connected to the USB cable, remove the the battery from the device a few seconds and reinsert it. If it boots--leave it to charge to 100% (check Options > Status for the percentage).
If it doesn't boot--try using a wall outlet charger if you are not. If you are connected via USB to the PC, make certain that you have Desktop Manager (or the USB device drivers) installed, which you can download below.
Make certain your USB cable is connected to one of the PC's main rear USB ports. Not a USB hub, nor a monitor USB port, nor a USB port on the front of the PC.
Download Desktop Manager from here: https://www.blackberry.com/Downloads...93E4F3BB068C22