If you use the Backup-SPSite PowerShell cmdlet to backup your SharePoint site collection and it fails for whatever reason, you may find that your SharePoint site is locked down in an read-only site. Visitors to the site may see a banner that indicates the site is under maintenance.
In SharePoint 2010 this situation could be remedied by unlocking the site in Central Administration (Application Management > Configure quotas and locks). However, in SharePoint 2013, if this situation occurs, the options to change the lock status will be greyed out.
In order to remedy this situation in SharePoint 2013, you need to change the site’s “Maintenance Mode” flag.
$Admin = new-object Microsoft.SharePoint.Administration.SPSiteAdministration('http://mysite/')
$Admin.ClearMaintenanceMode()
(via http://sharepoint.stackexchange.com/questions/64726/site-collection-locked-stucked-in-read-only-mode)