Sounds like a corrupted index in the mail program.

Possible solution:
<http://support.axon.com.sg/support/solutions/articles/3000034619-how-to-repair-apple-mail-inbox-empty-suddenly-all-empty>

—
Gary Nightingale

> On Apr 5, 2018, at 2:59 PM, Randy Zumwalde <zumwalde.rl@ehowe.com> wrote:
> 
> Hi,
> 
> I have a user who's account is set up using IMAP on all his devices using the Mail app.
> For months now, his web mail inbox is always empty. He's tried resending/refreshing from both ends and still nothing.
> Any idea why his web mail inbox would show up empty?
> 
> Thanks,
> 
> Randy
> 
>