SharePoint Site Error 503

SharePoint Site Error: Service Unavailable HTTP Error 503. The service is unavailable

** when we traced back the recent changes we could identify that from the CA Search Administration 'reset all crawled content' option was recently triggered which ended up with SSP page error 'Service Unavailable HTTP Error 503. The service is unavailable'. 

** other possible reason could be if you change the identity or account password either directly from IIS settings or if at all from web config file.

** another reason could be if you install another web based program or add feature.. unexpectedly causing IIS to stop the SharePoint services. for. eg: if you install AD on the same machine, OR added a new web site with a duplicate port.

make sure  that Web site protocols starts properly and the user account used to start Applicaion pool is correct and starts as well.

steps:

i.  Open up IIS Manager and click on Application Pools.

ii.  Locate your App Pool account and right-click on it and select 'Advanced Settings'.

iii. Click on the right of the Identity box to change it (A window will pop up).

iv. Click on Set and simply retype your App Pool Identity in there with the new password.