All my saved messages have disappeared - any ideas why and can I retrieve?

All messaged “flashed” for a couple of seconds when opening the SAVE folder and then vanishes. Using latest vesion on a 8.1 system with IMAP setting active. Messages are gone from other computers as well.

can you please check if your emails are still stored on the server by accessing your online mailbox?
What mail service are you using?

Thank you,