The email was not in the Drafts folder.
I had not sent the message and it was not therefore in the Sent folder.
I had also not deleted the message. (If I had clicked the red button on the draft, it would have asked me if I wanted to save it.)
The only thing I did in respect of the message was to flag the original message I was replying to while the draft was open.
I do not know if there was any interruption to internet or wifi but, even if there was, it should not have caused the draft to vanish.
It was a complex reply that I was drafting and had taken nearly an hour to write so it was very annoying that it was deleted outside my control.