This is a re-blog, I found the existing blog is incomplete. I added additional step to resolve the HTTP Error 503 for SharePoint 2013.
You might bump into the error message “HTTP Error 503. The service is unavailable” when you are having SharePoint 2010 or SharePoint 2013.
Make sure you start the necessary Application Pools in IIS.
Do check the account use for the Application Pool. Check on the Advance Setting. Then select the Identity.
Click on Set then re-enter the User name: and Password.