Avoid Office for Mac 2011 update 14.5.0
Microsoft released the Office for Mac 2011 14.5.0 update this past Tuesday and customers who’d installed the update on any version of Yosemite (OS X 10.10) began reporting their Outlook Main Window was invisible. Reverting to Office for Mac 14.4.9 by first uninstalling Office and then installing up to 14.4.9is so far the only supported fix.
For more info see:
‘Inbox Invisible’ plagues Outlook 14.5.0 users running Yosemite
Microsoft Answers forum discussion: View Windows Missing After Upgrade to Outlook 14.5.0/Can't send email.
Wait for an update from Microsoft. In the mean time, download and install the Microsoft AutoUpdate 3.1
https://support.microsoft.com/en-us/kb/3048681
Alternative
If you are unable to revert to previous version, you can download the Office 2016 Preview (free) and it will import your Outlook 2011 Identity. You can use Outlook v15.10 until there is a fix.
http://products.office.com/en-US/mac/mac-preview
You can download all the updaters for Office 2016 for Mac Preview on this link:
https://support.microsoft.com/en-us/kb/3048682
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