I was working on a project and all of the sudden my SharePoint site and Central Admin started giving me the error ‘Service Unavailable HTTP Error 503. The service is unavailable’ and I was not able to access either of them. I realized that I had just changed the password to the Identity account password and then this started happening. So here is what I did to take care of the problem (It worked for me and not sure if this is the most efficient way of doing it):
1. Open up IIS Manager and click on Application Pools.
2. Locate your App Pool account and right-click on it and select ‘Advanced Settings’.
3. Click on the right of the Identity box to change it (A window will pop up).
4. Click on Set and simply retype your App Pool Identity in there with the new password.
Kind of wierd but worked for me after giving me a good headache.
Nas
Nas's SharePoint Blog 