Bridge 11.1.0.175 Content Panel disappeared from application - not even visible in windows menu

By Gosh, you're right. I've used Bridge since v. 1 and have written about it and never noticed that (at least now) there's no Content Panel shown. 

Regardless, there are several solutions to your problem.

#1) Pull it back out. If you take any panel and pull it toward the edge, it will "disapear." You can pull it back out by hovering your mouse over the edge and you'll see a double-arrow, then mouse down and pull it back out.

Method 2: any time you've been moving the various Panels around back and forth, increase, decreased size and want to IMMEDIATELY get the window back to where you originally had it, just go to the Worksplace selection, right-click, and select Reset.

For the next time, glad you were able to find a solution that worked for you.

I found a solution but it is not a logical one.

I just turned other panels on and off.

Why is there no "content" panel in the windows menu? Just because you always need it?

Then what do you do when it disappears?

I recommend to put the content panel into the windows menu - it's bizarre that one of the two most important panels is not listed there.