Cause:

• Application Pool is in stopped state
• Password of Application Pool identity account is changed

First, check your Application Pool on IIS Manager, if its state is stopped then start it and re-check again your Sharepoint. In my case, i realized that application pool account’s password got changed.

Here are my resolution steps:

1. Login to Sharepoint server machine
2. Open IIS Manager (Start – Run – type “inetmgr” and press Enter)
3. Locate your Application Pool, right click on it and select Advanced Settings
4. Expand Process Model then select Identity. Click on the box on the right end of Identity.
5. Click Set and then give the user name, password and confirm password on Set Credentials box.
6. Restart your Application Pool.

Here’s Microsoft KB about this http://support.microsoft.com/kb/823552