1.Your previous connection to the network has been terminated by the system you were connected to.
2.Your network connection was lost.
3.Your network cable is not plugged in.
4.If uploading or downloading files, there might be something in the byte stream that is indicating to a firewall that is running real-time virus scanning that a virus is being detected, so the firewall shuts down the connection. If using http (80), switch to https (443).