Getting "Service Unavailable" error after changing Windows 2008 server Admin account:

When we change the Admin credentials, we must update the SharePoint Farm credentials and SQL server account as well...

1.  Open command prompt

2. C:\Users\Administrator>cd C:\Program Files\Common Files\Microsoft Shared\Web Ser

ver Extensions\14\BIN

3. C:\Program Files\Common Files\Microsoft Shared\Web Server Extensions\14\BIN>stsa

dm.exe -o updatefarmcredentials -userlogin Domain\loginname -password password

4. we will get the following error 

Value cannot be null.

Parameter name: farm 

Since SQL server was also running in the same windows authentication and we haven't updated it. So first update the SQL credentials

for the start-->run-->cmd--> Services.msc

Now change the SQL service password to the new one.

5. Now repeat the step 3. you will get it....:)