I've experienced this problem on more than 2 occasions whereby someone in the I.T. has changed User Account Profiles settings on Active Directory or, general log in policies have been changed which affect the application pool account details or the SharePoint Farm account if this has
been used as the Application Pool account.
Unfortunately, these changes do not automatically synchronise to Application Pools and need to be updated manually (hopefully this is corrected in SharePoint 2013).
Trying to log on to any of the Web Front End Pages will produce this error. Also trying on to Central Administration | Reporting so as to view administrative reports will also generate SharePoint error page messages.
To correct this, Log on (with an administrator account) to your SharePoint Farm's main Central Administration Console is i.e. the web application server and perform the following:
1. Click on Start | All Programs | Administrative Tools | Internet Information Services (IIS) Manager. Please don't choose 6.0 Manager
2. Internet Information Services Manager (IIS) will now launch. On the left hand focus, click on Start Page | Your SharePoint Server | Applications Pools.
3. The Application Pools focus will now be displayed. Select the Application pool for the web application that need their account details (i.e. the identity) to be changed. Next, click on the Advanced Settings link on the "Actions" focus.
4. The Advanced Settings section will now be displayed. Scroll down and select the Process Model section and double click on the Identity. A grey box will then be displayed. Click on the grey box.
5. The Application Pool Identity will now be displayed. Click on the "Set" button
6. You will now need to re-enter the Identity including the domain name and the password details that were updated in Active Directory.
7. Click on OK and all of the page should now work on refresh. In future, you might want to consider placing those accounts into a seperate Organisation Unit in Active directory as well as updating your I.T. strategies regarding changing user names and passwords. Also, ensure that theFarm and Application Pool accounts passwords are set to never expire.