Outlook Add-ins might preventing Outlook to open up.
Add-ins on Outlook might generate a wide range of errors or conflicts resulting in warning messages, Outlook restarting and or freezing.
On Outlook, go File (Upper left side) > Options
Go to Add-Ins Tab > COM Add-ins and click GO.
Clear all the add-ins checkboxes, add them one by one each time loading 3 of them, restart Outlook, check if the issue is still present and repeat the above process till you spot the add-in that is causing the problem.
Another software is generating a conflict.
Run a Selective Startup to determine if another program is interfering with Outlook
When we start Windows, numerous applications and services inevitably start and keep running in the background. These applications and services can interfere with Outlook. The Selective Startup or "clean boot" can help us to detect issues with conflicting applications.
The Selective Startup option in System Configuration (Startup Tab), enables services and default programs to turn on or off separately; This will help to determine if the problem re-occurs next time we restart Windows.
When we select the check box in System Configuration, the correspondent service or startup program will run at the next Windows restart. If the check box is cleared, the correspondent service or startup program won't load up at the next windows restart.
Go to Control Panel >> System and Security >> Administrative Tools, >> double-click System Configuration to open it. If you're asked for an administrator password or confirmation, type the password or provide confirmation.
Click the General tab, click Selective startup, and then clear the Load system services and the Load startup items check boxes.
Mark the Load system services check box, click OK >> Restart.
If the error occurs again after rebooting, follow the steps below:
a) Identify the system service which generates the problem.
b) In System Configuration, click the Services tab, click Disable all, select the check box for the first service that is listed, and then restart the computer.
If the problem doesn't reoccur, then you can exclude the first service as the cause of the problem.
c) With the first service selected, select the second service check box, and then restart the computer.
d) Repeat this process until you reproduce the problem. If you can't reproduce the problem, you can exclude system services as the cause of the problem.
e) Spot he startup item which creates a conflict (use the same strategy as above followed with “Services”)
f) In System Configuration, click the General tab, and then select the Load startup items check box
g) Navigate to the Startup tab, mark Disable all, choose the checkbox for the first startup item that is listed, and then restart the computer. If the problem doesn't reoccur, you can exclude the first startup item as the cause of the problem. With the first startup already checked, select the second startup item check box, and then restart the computer. Replicate those steps until you reproduce the problem.
The process is a little bit tedious, but allow us to identify the add in preventing Outlook to open up.