SharePoint site shows ‘Service Unavailable. HTTP Error 503. The service is unavailable’

I haven’t started the SharePoint server for a while, and when I start it up, I find that Central Administration can’t access it, which displaysService Unavailable HTTP Error 503. The service is unavailable,

How to Fix this Issue?

    Go to Start >> All Programs >> Administrative Tools >> Internet Information Services (IIS) Manager. Locate your App Pool account(such as SharePoint Central Administration v4, the third line on the right panel, see below figure) and right-click on it and select ‘Advanced Settings’. Go to process model then Identity Click on the Identity >> Set and then give the user name, password, and confirm password.

The reason may be that the account password has expired and needs to be reset.

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