You may receive "Service Unavailable" error while connecting to Team Foundation Server(TFS). I was getting this error from yesterday whenever I was trying to connect to TFS. Finally today I found the reason and solution. I had changed the password of Administrator user which I had used to configure TFS Application Pool. This has stopped the application pool as the account which is running the TFS Application Pool has old password. So one has to change the password in TFS Application pool. To change password in application pool, click Start - Run and type inetmgr. This will open IIS. Now select the TFS application pool.
Right click on TFS Apppool and select properties. Select Identity tab. Here you have to enter a new password for this account and click ok. Restart the IIS and try to connect to TFS. This time you will not get the "Service Unavailable" error.
You will get this same error if you have deleted the user which is used to configure TFS Application pool. Here is the good article from Microsoft.
Happy Programming !!