Another thing that would cause your laptop to over heat is that it is not placed in a right position. you always have to keep your laptop on a flat surface especially when you are going to use it for continuous hours.

As David mentioned your laptop's battery might have died during the process which would cause this issue. Another case your laptop could have faced is that the screen froze during the downloading process which would cause this black screen to occur. However, try restarting your laptop by holding down the power button till it is completely off then repress it to start it again it should resume the download.