Network Disabled on Ubuntu 18.04
It is likely that the network-manager state got corrupted. Open the file /var/lib/NetworkManager/NetworkManager.state. It should look something like this:
[main]
NetworkingEnabled=true
WirelessEnabled=true
WWANEnabled=true
Change any from false to true to re-enable networking. It may work better if you first stop NetworkManager:
sudo stop network-manager
And start it again once done.. Or reboot
This happened occasionally after wakeup in Lucid. In Maverick this should be fixed.