HTTP Error 503. The service is unavailable
This is a common issue which happens to the websites hosted on IIS.
This occur when the application pool of the hosted website may be stopped.
The error message will be shown as in below figure if the application pool is down.
On Run Command Window, type inetmgr to open IIS and Select your website.
Right Click on the Website --> Manage WebSite --> Advanced Settings.
A new windows will be opened where you can find the settings for that website.
On the General Section, note down the name of Application Pool.
Now on the left panel of the IIS, click on the Application Pools Node at the top.
Select the earlier got Application Pool from the Features View.
You can see the application pool is stopped now.
On the right Action panel, click on the start link for starting the application pool.