Shutdown and disconnect all external drives. Move as close the Router as possible. Have in hand the Password for the wifi Router. Now, plug into charger. Restart the computer and Immediately hold the three key - Command + Option + r -. This is called Internal Recovery Mode. Watch closely for Wifi Icon to appear and click it. You will have to Input the Wifi Password. Once the connection is established an Apple Globe will appear and load slowly. Depending on the Speed of your connection and Apple Servers - this can take some time to Fully Load. Once it does - it will present much like in internal Recovery Mode. Choose Install macOS and it will download and install macOS.