I had an issue on Friday where all my emails disappeared on my ntlworld account. When I opened either the Inbox, Set Items or Trash, they showed the contents which about 2 seconds later completely disappeared. They were gone from my phone on Outlook too.
I could send emails but didn't appear to be able to receive any.
I contacted Virgin who led me to the "email rules" which is found under "settings" (the little cog wheel top right), "email", "filter rules". There I found that two "rules" had ben added which forwarded all my emails, past and incoming, to a different email address and deleted them from my account. It seems my account had been hacked and these rules added.
Once I changed all my Virgin and ntlworld passwords and deleted the "rules" all is fine going forward so far. Sadly there seems no way to retrieve the lost emails.
Fortunately I have a habit of creating folders within the Inbox for things like gas, electricity, holidays, insurance, receipts, orders etc., anything I think is important, and moving relevant emails into the appropriate folder. Those emails appear to have survived the hack.
Hope that helps.