If a user is attempting to click on the Outlook icon and it remains unhighlighted, it may just be due to the application not closing correctly. Below is some information on how to quickly nip this issue in the butt.
Problem:
You attempt to click on the Outlook Icon, and nothing happens. It may flash, but mostly appear as if it didn’t register your command 
Fix:
One of the most likely causes of this issue is that the Outlook Process is running in the background, and we have to ensure we end it. To do so, hit Ctrl + Alt + Delete or alternatively open the windows bar and type in “Task Manager” in the windows if you are remotely helping a user and are unable to use the keyboard shortcut. Once open, select the processes tab up top and search for OUTLOOK.EXE
End the process 
and reopen outlook. This time, it should open without any hiccups. Good practice is to open Outlook again in Safe Mode, which can disable anything that may have caused an issue the last time it opened. You can disable Add-ins as well that may have been causing the issue.
As always thanks for reading! -H
