Trying to diagnose my sudden lack of wifi, but I have no idea what this icon means.
If you had wifi and then suddenly it disappeared then you should first try switching off and on wifi and if doesn't work then just restart network manager with
sudo service network-manager restart.
If you still have no internet then just restart your computer.
I have the same problem sometimes. I couldn't find out better solution.
There are various forms of this:
Losing wifi connection is a Ubuntu 16.04 wifi issue: 16.04 LTS wifi connection issues.
The change of icon is related to that too - but the form of the icon may vary depending on the icon-set.
It became a rare occurrence for me after I did this: https://askubuntu.com/a/768268/47206
When it still happens I do
service network-manager restart (asks for password anyway). That command can be run from a shortcut or added into a
~/.local/share/applications/restart-wifi.desktop file and run from slingshot.