Whole Inbox disappeared

Been having warnings pop up on screen, now my entire 4 years worth of business emails inbox has just disappeared!

Warnings are like this:
[IMAP] Unable to get quota information, server responded with an error: Client tried to access nonexistent namespace. (Mailbox name should probably be prefixed with: INBOX.) (0.001 + 0.000 secs)…

[IMAP] NO [OVERQUOTA] Not enough disk quota (0.003 + 1.174 + 1.177 secs).

Can anyone help?

First thing you should do, as a Pro License user, is open a support ticket directly with eM Client. You are entitled to VIP Support, and they will be able to help you either by email or remote connection to your computer.