My suggestion was to rebuild your solution files.
In the root folder of the projects, copy Assets and Project Settings ONLY into the new directory. One of your scenes opens from here, Unity will import assets and build a scene.
After opening, go to Edit â Preferences â External Tools and make sure that Visual Studio 2015 is selected as an External Script Editor , and then go to Assets â Syno MonoDevelop Project (Ignore the name MonoDevelop!)
See if this works for you.
The inability to follow the guide here and point out which version of Unity you are using as there are various steps for pre 5.2 Getting Started with Visual Studio Tools for Unity
And if this does not work, it will be a problem when installing Visual Studio. I had it before even on a new installation. So try reinstalling as a last resort.
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