Menu: Tools -> Options -> Projects and Solutions -> General
This one is interesting and the main reason I'm showing it to you is so you know how to turn this back on if it ever gets turned off. I love the Visual Studio team but every once in a while I think there is a sadist who joins them just long enough to get a weird option put in and then leaves the team. This would be one of those options.
If you Tools -> Options -> Projects and Solutions -> General you will see many options there. Locate the "Show advanced build configurations" option:
Simplified Build Configurations
So what does this option do? Well, when it is turned OFF, it uses Simplified Build Configurations which involves several changes:
In Simplified builds, the Configuration Manager is not available. At all. Ever. Seriously it's just disabled (or removed completely) from all areas:
The practical implications of this are you can't do any custom build configurations. Which makes sense with a SIMPLIFIED build configuration option. So when you just don't see this anymore or it's disabled that is usually a clear sign that you have turned off "Show advanced build configurations".
In a Simplified Build scenario, each time you press F5 or go to Debug -> Start Debugging it will create a Debug build of your application. This is expected usually so not a real surprise.
The real shocker is there is no obvious way to create a Release build. With Simplified Builds you create a Release build by going to Build -> Build Solution (CTRL + SHIFT + B).
Advanced Build Configurations
I could do an entire series on advanced builds but I'll just keep it to the basics for now. Essentially when you choose "Show advanced build configurations":
It gives you access to the Configuration Manager so you can actively switch between build types without having to remember some arcane steps to do it:
As an added bonus, it gives you the ability to make custom builds with your own special configuration options specified for the build.
In short, there are lots of good reasons to show the advanced build configurations options and really no good ones that I can think of to turn it off. So just leave it on.