http error 503 the service is unavailable

I got the Error "http error 503 the service is unavailable" when I was trying to open SharePoint 2010 Page in IE with IIS 7.


I tried for a while to figure out the problem, tried restarting the IIS App pool , nothing worked out.

When I check in the event viewer, I saw below details under

Event Viewer - Windows Log - System - check under WAS (Source)

Error
Application pool SharePoint Central Administration v4 has been disabled. Windows Process Activation Service (WAS) encountered a failure when it started a worker process to serve the application pool.
Warning
The identity of application pool SharePoint Central Administration v4 is invalid. The user name or password that is specified for the identity may be incorrect, or the user may not have batch logon rights. If the identity is not corrected, the application pool will be disabled when the application pool receives its first request. If batch logon rights are causing the problem, the identity in the IIS configuration store must be changed after rights have been granted before Windows Process Activation Service (WAS) can retry the logon. If the identity remains invalid after the first request for the application pool is processed, the application pool will be disabled. The data field contains the error number.


I observed that, I was getting this error after changing my windows password.

After doing trial and Error, I found the solution

Solution:
Go to Inetmgr
Select the Application pool, click on Advanced Setting
Select Identity; Reset your account name with your new password.
Restart the application pool, your application will start working.


Observations:

I am not sure whether it is a bug with IIS 7 or SharePoint 2010 application.

We may have to take care of the above steps, when you create application with your Windows credentials and after creating the application, if you change your windows password and try to access the application.

In my case, the application was still working even after changing the password, but I started getting error, when I go to Central Administrator and try to create new application, I got error saying Authentication failed, after that, I cancelled the process of creating new application.

After that, When I try to open any SharePoint application or Central Admin page, I started getting the error "http error 503 the service is unavailable"