SharePoint 2010 error: HTTP Error 503. The service is unavailable.

 This might me due to the following reasons.

SharePoint administration site is in stopped state in application pool in IIS.

Or 

If SharePoint administration site was running under a user account and after installation password was changed for that user then that might be the case for this error.

Solution: In IIS go to application pool then go to the advanced settings of SharePoint central administration and change the pass of the account to current.