Method 1: Start Outlook in safe mode following steps mentioned below and verify whether issue re-occurs.

* Hold Windows key + R.
* Copy and paste, or type the following command in the Open box, and then press Enter
Outlook /safe
Note: There is a space between Outlook and '/'.

If Outlook works as expected in safe mode, disable the add-ins by following steps mentioned below and check if it works as expected.

a) Click on File menu, click Options, Add-ins,Go button besides Manage: Com-in Add.
b) Check if there are add-ins listed, clear the checkbox.
c) Close the Office program and restart it.
d) Enable each add-in one at a time, restart the Outlook, and repeat the above procedure. If the issue reappears again, it will help you to determine which add-in is causing this issue.

Outlook 2016 Hangs on "Starting ..." - Microsoft Community