Inbox emails have disappeared

All emails disappear to not be seen again after they flash up in the INBOX. I have shut off the so far useless unified inbox because nothing would show there.

I am new to K-9 but a new cell phone has required a new program. I have configured, used and saved every email since 1995. So I am not a newbie.

I require POP3 and the app uses POP3. In frustration I have also connected to IMAP no success. The emails just do not show up. I have read emails this morning but then need to return to them for more work and they are gone. Cannot find a trace of them in deleted / trash, archive, or inbox folders.

What exact version of K9 you are using?
What mail provider?
Do you also use an additional mail client on computer, e.g Thunderbird?

You say you worked on mail, and when you came back later mail is gone. Does your mail provider provide a web interface where you can check directly on server? Or in another mail client.

Sorry, cannot help, still need to understand issue better.