Question mark icon instead of wifi icon

Settings > Privacy > Connectivity Checking

Set it OFF. (Then, restart your WiFi connection, thanks to @Dante's comment below).

This is the same fiasco as Microsoft's NCSI (Network Connectivity Status Indicator), a false negative to try ruin your mood.

And, yeah, it would be checking some URL like that one.

I've had that issue with other versions of Ubuntu. I had similar Theme issues after upgrading to 18.04.

Change the icon theme. You can do this with Gnome Tweaks.

It's not installed by default. Install it with:

$ apt install gnome-tweak

From there click on Appearance -> (click) Icons. Then select a different theme.

I've seen occasions where changing to a different theme and changing back sometimes resolved a glitch that was happening with the current preferred theme.

I solved it differently because I am able to find the root cause which is kind of different from what you get.

Open `log' and search for 'network', I see following error message

... result="fail" reason="Connection 'Ethernet connection 1' is not available on device ens33 because device is strictly unmanaged"

This is how I solved it:

Open

sudo nano /etc/NetworkManager/NetworkManager.conf

and change the line managed=false to managed=true

Then,

sudo service network-manager restart