One of the most frustrating things that you’ll encounter with Lightroom is when your entire title bar for the application disappears. You’ll see your menu but everything else including the Close, Minimize, Maximize and other buttons will be missing.
The problem is that you’ve pressed the F key to get to full screen mode.
The solution is to press the F key repeatedly until the screen returns to normal. Too easy? Maybe but it sure is frustrating when you don’t know what happened and how to fix it.