My Add-on Panel / Window / Tab is missing My add-on UI (window/tab) is missing after I moved my Blender folder to other folder. I am using a Mac and Blender 2.79. 

I've tried to move the folder back and to reinstall Blender. The results are still the same:
It is likely that you have an add-on that is not working as it should. Try looking at your /Users/[username]/Library/Application Support/Blender/ folder. If you find the add-on that you suspect might be the issue in the scripts folder of your version folder try removing it. You could delete the entire folder with your current version number(maybe backup it first) to restore the default user preferences. It should be recreated the next time you save user preferences.