Accounts disappeared from Outlook Navigation pane

I've been using Outlook 2011 for Mac on a Macbook Air for about 3 weeks and have four email addressees which initially appeared in the navigation pane, being expandable to show the Inbox, Drafts  etc for each account.  A few days ago they just disappeared!  I didn't consciously do anything to make that happen. I'm still receiving emails from all four accounts but I only have one composite inbox, drafts etc showing. How can I get the display of individual accounts back?