Hi Shuan,
This error is very generic but I used to see it a lot when hosts were trying to run .NET 2 and .NET 1.1 code in the same application pool. This is why Asad suggested a dedicated Application Pool.
Generally, this error means the Application Pool is not started and it can't start and the most common cause of that is mixing .NET framework versions. Most modern hosting control panels allow you to change the application pool your site is using, but I don't who is hosting your site or what control panel they use so I can't tell you how to do this for your site. If you can get me that information, I might be able to help you further. Also, if you know what Server OS and IIS version is running, that would help a lot.
Joe A. is absolutely correct about your host being able to tell look in the Server Application Log for errors related to the Application Pool your site is using.
HTH,
Joe D.