Microsoft released a Mac Office 2011 14.5 updater on 5/12/15 that broke Outlook on any Mac running OS X 10.10 Yosemite where the user had set Outlook to show the contents of a subfolder (turned triangle down next to a folder). After applying this 14.5 updater, the main Outlook window would not open on affected Macs.
Microsoft silently released a Mac Office 2011 14.5.1 updater to fix this problem on 5/19/15, but as of the publish date of this blog post, this update has yet to show up using Microsoft’s AutoUpdate (Help->Check for Updates). It currently must be manually downloaded from https://www.microsoft.com/en-us/download/details.aspx?id=47307
My recommendations are: Regardless if you’ve running Yosemite or not, if you haven’t updated Office 2011 to 14.5 yet, hold off until 14.5.1 shows in Microsoft’s AutoUpdate. If you are running Yosemite, and have already updated Office and Outlook isn’t working properly, then manually apply the 14.5.1 update to fix the problem.