It should be in the disconnected maibox list
you can right click and select re-connect
or
if it is not there, then
see on which computer the user used / accessed his mailbox the last time ;
you can then browse to %appdata% on that computer and go into "Local" instead of "Roaming" then Microsoft > Outlook > and there you will see the OST File, you can convert OST to PST and then re-import his mailbox into exchange.
