Are you still running Outlook 2010?
Is it now opening in safe mode after the December 2015 updates were installed?
What is Safe Mode
Safe Mode is a fallback mode of Outlook (and other software) when it cannot run in normal mode because of some error. Normally, this would be caused by a third-party add-on to outlook such as Anti-Virus, Anti-Spam, or Calendar Syncing. Safe mode is useful for troubleshooting a problem because the bulk of the software can continue to operate when a small sub-function has failed.
In the case of Outlook, this means that you can still get to your calendar, send and receive email, etc. but features such as the ability to customize and save how your workspace looks and which folders open by default, as well as all plugins, are disabled.
However, on December 8, 2015 Microsoft released update KB3114409 which “broke” Outlook for many people. While Microsoft pulled the patch, the damage was done.
Ironically, the patch was supposed to add administrative support to PREVENT Outlook for booting into safe mode, so that plugins are always forced to run. Instead, it seems to have had the opposite effect.
How to fix it
Luckily, Microsoft also released a “patch for the patch” in the form of KB3114560 on December 16, 2015.
This update is available for manual download and installation from the Microsoft Download Center.
Download update KB3114560 for 32-bit version of Outlook 2010- Download update KB3114560 for 64-bit version of Outlook 2010