Had a call from someone who had clicked the WIndows 10 free upgrade option and then found themself offline after the reboot. So they were unable to get online and check for updates or found out what had gone wrong.
It was a Dell PC that used an Intel 82579LM Gigabit network adapter. There was a red cross in the bottom right indicating that the network LAN was disconnected, with no internet available.
On Windows 10, go to the Search box in the bottom left by the Windows Start icon, type in Device ... you should be prompted with a link to Device Manager (in Control Panel) as the best match. Start the Device Manager and look for the Network Adapters section.
In this case there was an Intel(R) 82579LM Gigabit Network Connection, similar to the one shown in the picture. Right-click on that entry and choose Disable, give it a moment and the right-click again and choose Enable. This effectively turns the network connection off and back on again. After a minute or two it came back to life, emails arrived and Internet Explorer and Chrome were working again.