I had a strange problem with wifi on my brand new installation of Elementary Linux 5.0.
First there was no problem, everything worked out of the box, but after some hurs suddently the wifi disapeared.
I could get the wifi icon active but in the Network Preferences wifi was stil marked as disabled?

The solution is very simple:

Write the following in a shell: 

The wifi works, but with the radio turned of, it cannot discover my hotspots. After using the command, everything worked just fine again.

If you dont want to write the command every time you boot, you can ad the line to the file (might be empty) /etc/rc.local