An alternate solution to the problem of not having upgraded to April 2013 CU is a simple powershell script that uses reflection to set the value of the internal 'MaintenanceMode' property:-
$site = Get-SPSite http://urltofreakinlockedsite/
$site.GetType().GetProperty("MaintenanceMode").GetSetMethod($true).Invoke($site, @($false))
This approach has the advantage of not needing to touch the database. Having used .NET Reflector on the April 2013 CU version of Microsoft.SharePoint.dll, this is in fact what SPSiteAdministration.ClearMaintenanceMode() does!
You will need to run this on the server, and as a farm administrator with write access to the content db.
This will work on pre- and post-April 2013 CU versions of SharePoint 2013