Service Unavailable error in SharePoint 2013

I've just started SharePoint 2013. After the deployment, got instantly this error. Here's the solution.

I created a new project using Visual Studio 2012, then deployed a simple Visual Web Part.

Then suddenly got this:




Then soon I found out that, the application has stopped working, so I launched "IIS Manager" there got to the Application Pools, and Started SharePoint site.

After that everything worked.