My Zimbra chat doesn't work anymore

[Lums]

Hello,
I use zimbra's chat with my colleagues but I've been facing problems since yesterday.
It starts always the same way : my contact appears to be typing for hours (though he is sending messages to me that I don't receive) then he appears as off-line.
And now, I appear on-line but the messages my colleagues sends me appear as gray and unread but I don't receive any.
What can I do ?
Thank you very much,
Best regards,

Lums

[Lums]

I forgot tot mention that from the beginning, eveyday when I open the chat, I can't see the previous messages from the day before for instance. It's always as if I'm sending a messag eto my colleague for the first time.