I do have a SharePoint (2013) site collection that is stuck in read only mode due to a backup that was either interrupted or didn't terminated properly.
I'm unable to unlock from :
  • Site Quotas and Lock in Central Administration (option is greyed-out)
  • Stsadm : operation completes successfully but no change
  • Using PowerShell : same behavior as stsadm
I reviewed the associated content database to ensure it was not in single user mode neither in read-only which was not the case. Disk space is not an issue neither.
 
SOLUTION!
PS C:\Users\root> $Admin =  new-object Microsoft.SharePoint.Administration.SPSiteAdministration('http://root.toto.com')
PS C:\Users\root> $Admin.ClearMaintenanceMode()
Also

I found this and successfully unlocked the site collection!

https://sharepoint.stackexchange.com/questions/64726/site-collection-locked-stucked-in-read-only-mode

This is the PowerShell.

$Admin =  new-object Microsoft.SharePoint.Administration.SPSiteAdministration('Your Site Collection URL')
$Admin.ClearMaintenanceMode()