Pre-requisites 

The IIS server needs to have one feature enabled for this to work. It's called "IIS redirect" and you can turn it on in the "Roles and Features" Panel of the Windows Server Manager interface. Details here.

Setting it up 

1. Download this zipped folder.
2. Unzip it in the /wwwroot directory in a "Maintenance" folder.
   
   
   
   
   
3. Edit the <article> section of the html document to fit with the client's need or template. The "mailto" link lets you define a contact email address. Don't forget to edit it as it currently points toward "someone@example.com".
4. Open the IIS module and navigate to the Webprod app.
5. Set the "HTTP Redirect" parameter to the name of the unzipped folder (here: Maintenance) and the "Status Code" to "Temporary (307)".
   
6. Check the "redirect requests to this destination" checkbox.
7. Hit the "Apply" button.
8. Do not forget to uncheck the checkbox (and hit "Apply") when the server is back up and running!