Make sure you are showing all files. There is a button at the top of the Solution Explorer called "Show All Files". To see this button, make sure that your project is selected in the solution explorer.
Show All Files
![enter image description here](https://i.stack.imgur.com/Rwli3.png)
When this option is active, the file should be there, just grayed out. Right click it, and select "Include In Project".
Include In Project
![enter image description here](https://i.stack.imgur.com/c8eJf.png)
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