Service Unavailable is very common error any one can face. And there could be several reasons for the same. This issue may occur if the server that is running Microsoft Internet Information Services (IIS) 6.0 or higher is could be a domain controller. The problem occurs because the Application pool is using the NT AuthorityNetwork Service account, and the NT AuthorityNetwork Service account may not have permissions to access the required folders. You can find more information about “Service Unavailable” error here.
Same thing applies to SharePoint as well, as it is built on ASP.NET 2.0 (SahrePoint 2007) and it is a web based collaboration solution so it will also have its entry in IIS. And of course it will also need permission on certain folders. Due to this reason we use admin user account credentials for its application pool. As best password policy followed by organization, after certain time period is elapsed the account’s password will change. And when we will change the password for that account, we also has to change the password used to configure the application pool for SharePoint sites including Central Administration.
It is advisable to use a separate user account for application pool associated with the SharePoint web application(s), also ensure that account does not have log in permission on the system and it should only have log in as service permission and the account does not have any lockout/expiration settings.