If you were trying to run a back up or stop the backup but your site is showing currently in read only mode and option to unlock at central admin is also disabled, below is the powershell commnds you can run to make the site active again.
PS C:\Users\root> $Admin = new-object Microsoft.SharePoint.Administration.SPSiteAdministration('http://YourSite')
PS C:\Users\root> $Admin.ClearMaintenanceMode()
One ran, refresh the site and you will see the site in full active mode.
