Sharepoint - We apologize for any inconvenience, but we've made the site read only while we're making some improvements

I have came across many such scenarios, this happens mainly due to powershell command being interrupted. To resolve this you can use following powershell script

$admin = new-object Microsoft.SharePoint.Administration.SPSiteAdministration ("http://sharepoint/sites/sitecollection") $admin.ClearMaintenanceMode()

Once you execute this command it will make your site normal. Hope this helps.