Service Unavailable HTTP Error 503. The service is unavailable

Solution1:Check for the Application pool status. If it is stopped, start it.
If it is stopped again..
1. Select application pool --> Advanced settings --> set Identity property value to admin account details.
2. Start the application pool again.

Solution 2:
1. Run following cmdlet:
           Set-SPManagedAccount - UseExistingPassword -Identity DOMAIN\SPSAdmin
2. Start the application pool