Reset DefaultAppPool and Default website on IIS 8.5

When I open the URL from any other system the IIS default page was loading perfectly. Then I tried to add my web app in the application pool, which messed up lots of settings but it did not work and i removed it from the pool.

Now, when I open the URL it shows this error instead of displaying the default IIS page and defaultAppPool is stopped.

I want to reset IIS to its default settings. Please help!