Cause: Driver conflict or device not in EDL mode. Solution: Re-install Qualcomm drivers. Check Device Manager – the COM port should be active. Disable "Qualcomm MSM MUX".
Cause: The phone exited EDL mode (battery too low or timeout). Solution: Re-short test points or use a deep flash cable (EDL cable).
Once you have the loader, you need the proper tools and firmware.