Present a secured site maintenance page.

To configure the content to come from only the specified destination directory, use the following syntax:
appcmd set config /section:httpRedirect /childOnly:true | false
By default, this attribute is false, but you specify true for the childOnly attribute. To do this, type the following at the command prompt, and then press ENTER:
appcmd set config /section:httpRedirect /childOnly:true

Assuming you have IIS Scripting installed, open an elevated PowerShell:

import-module webadministration

Consider stopping all sites except the Offline one:

Get-ChildItem IIS:\Sites | Where {$_.Name -ne "Offline"} | Stop-WebSite

when the SQL-Server is back up, start them up again:

Get-ChildItem IIS:\Sites | Where {$_.Name -ne "Offline"} | Start-WebSite

For FTP sites, the commands will show an error. You cannot pipe an FTP site to a Stop-WebSite cmdlet, but it still works for all the web sites.

For sites that are not running alright, consider to exclude them:

Where {$_.Name -ne "Offline" -and $_.Name -ne "foobar.com"}

If there is no PowerShell cmdlets for IIS installed, you can explore appcmd.exe