So I was deleting stuff off my computer, and turns out I might've deleted something, because it says "There is no application set to open the document", and all .fla files are just blank white documents when I look at them, it doesn't look like one of those red flash files with an 'F' on it, it's just is a blank white document. I don't know what I deleted, but it's not working.
In Windows you can tell it to open a specific file type with any application you wish. You may have reset this preference in your OS.
On Windows, right-click to select the .FLA document, select "Open" if it's bold at the top of the list (which indicates Windows has no default for it), then select Flash Professional from the list of application that show up. If Flash Pro does not show up you can use the browse button to navigate to your install, which is by default (for CC 2015) in this location: C:\Program Files\Adobe\Adobe Flash CC 2015\ and select the file Flash.exe. That will properly assign that file extension back to Flash.
If you right-click and there is no bolded "Open" option you may have an "Open with" option, which means at least one application is set up for that extension, but if might be incorrect. If you use "Open with" and do not see Flash Pro listed there like this then you can use the "Choose default program..." option and follow the steps above: