I guess it was the Unread Pm notification
You mean the bar along the top that shows when there's a new PM?? That's a default feature. However I know there is a plugin for it but if that's the last thing you added before it happened, try that.
ok, i was completely fed up with it. So I uploaded a new mybb script & used the database backup. Therefore, all normal for now.
Thank you all for your help.