HTTP Error 503. The service is unavailable. App pool stops on accessing website

One possible reason this might happen is that the Application Pool in IIS is configured to run under some custom account and this account either doesn't exist or a wrong password has been provided, or the password has been changed. Look at the advanced properties of the Application Pool in IIS for which account it uses.

Also the Event Log might contain more information as to why the Application Pool is stopping immediately on the first request.

Ok, I have another solution for one specific case: if you use WINDOWS 10, and you updated it recently (with Anniversary Update package) you need to follow the steps below:

1. Check your Windows Event Viewer - press Win+R and type: eventvwr, then press ENTER.
2. On the left side of Windows Event Viewer click on Windows Logs -> Application.
3. Now you need to find some ERRORS for source IIS-W3SVC-WP in middle window.
4. Probably you will see message like:

The Module DLL >>path-to-DLL<< failed to load. The data is the error.

5. You have to go to Control Panel -> Program and Features and depending on which dll cannot be load you need to repair another module:
   • for rewrite.dll - find IIS URL Rewrite Module 2 and click Change->Repair
   • for aspnetcore.dll - find Microsoft .NET Core 1.0.0 - VS 2015 Tooling ... and click Change->Repair.
6. Restart your computer.

For my situation is that my login password changed, while the application pool still uses the old one. So just click the "Advanced Settings" of your application pool and reset your "Identity".