IIS HTTP Error 503. The service is unavailable

This error might happen if the application pool for the web site account password is changed and not updated,

Open the IIS Manager

Go to Application Pools

Select the Application Pool related to the website.

Right Click and select Advanced Settings

under Process Model Select Identity

Click on ... button next to the the user name

Click on "Set"

Enter the user name and new password

Click Ok and close the popup windoos

Start the application Pool