This is on Ubuntu 16.04.3 LTS.
I had an issue with my wireless card/drivers recently (outlined here) which I couldn't fix. I booted from a Live USB and still no networks were detected. Then I chose to install Ubuntu from the Live USB, and my wifi began to work! I erased my disk completely and installed Ubuntu from scratch.
After installing, no wireless networks were detected again.
ifconfig shows only a loopback device as before.
I repeated the process with the same results. Wireless works only during the install process. The only thing strange I noted is that after installing, I'm prompted to restart. It brings me to a black terminal-like screen with a flashing underscore, and I have to force shutdown my computer. This happened every time.
I followed the suggestions in the post Internet worked during install, but not after with no success.
Thanks for any suggestions.
Reinstalling Ubuntu from the Live USB and choosing to install third party software but not download updates while installing allowed me to access wireless after the install was complete.
sudo apt-get update and
sudo apt-get upgrade performed as expected, but
sudo apt-get dist-upgrade caused bcmwl-kernel-source to crash. It appears upgrading the kernel causes the driver to crash.