In your menu in your top left corner, you must click on Render (Between Tools and Connect), then on "Render Setting". It'll open the render setting tab.
Then click on General section. You'll see a bunch of option with pixel size, aspect ratio, ect... In the top right corner again, you'll see a box. Something will be written there. (Probably "Custom". But it can be something else.)
Click on that and select the first option "Active view port". It'll reset your aspect ratio rendering to default. (which is your scene view port.)