messages gone, seems to be expunging when i send/receive

from no where all my previous messages have gone, no in box, no out box, no previous message sent, just have On This Computer where they were. to the right of the send/receive button, all is greyed out except for the cancel the current email button. when clicking on the send/receive button a pop3 message flicks up very briefly and I think it says something about expunging something or other, but it is so brief that i have no time to read it.