This is a known issue, but so far we haven't been able to reliably reproduce it here in order to be able to fix it.
Restarting ConQuest restores the display and brings the tabs back. The quickest and most reliable way of restarting ConQuest on any type of system (ie, this works on remote sessions, which closing and re-opening may not, depending on how it has been configured) is to go to Options and then click OK without changing anything.