Mcapelloooo, you were dead right! Sure enough, this morning (Australian time) my Sent folder came back to life and started keeping a record copy of a few test emails that I forward late last night. Now all seem to be as it should be. But why the hassell in first place, I ask? And it's not only me but possibly many hundreds, even possibly thousands, of people across the globe experiencing the same issue.
So thank you, and also to jsf63, for taking the time to reply, much appreciated.
The only remaining issue, and it really is only a minor one, is that now I have two Sent folders! The one that is third on the Mailbox list after Inbox, then Drafts, is the working one. But then I also have a second Sent folder that is in 8th position on the Maibox list that doesn't get populated with my sent emails but the 'system' won't allow me to delete it! When I try it brings up a message, "Unable to Delete Mailbox The mailbox could not be deleted in the server." But as I say, just a very minor issue and this empty Send folder I can live with!!