1. Outlook does not fully exit when you close it

In this case, Outlook does not exit completely when you close its main window. This is usually caused by a third party add-in which prevents Outlook from exiting. You can usually recover the missing toolbar by following these steps:

• Close Outlook
• Open the Task Manager
• Switch to the Processes tab
• Find OUTLOOK.EXE in the “Image Name” column 
• Select it and click “End Task”
• Wait 5 seconds, then start Outlook again.

That should bring your missing toolbar back.

If this keeps happening to you, you might want to disable some of the add-ins you have installed, to find which one is causing this issue.

How to disable add-ins in Outlook 2010
How to disable add-ins in Outlook 2007
How to disable add-ins in Outlook 2003
 

2. Outlook disabled the add-in

If the above steps did not help, please try rebooting. If you still do not see the toolbar, it is possible that Outlook disabled the add-in. In that case, you can re-enable it using the following steps:

Outlook 2010:

• Select the “File” ribbon tab
• Click “Options”
• Click “Add-Ins” in the left column of the Options window
• Select “Disabled Items” in the “Manage” drop-down and click “Go”
• Select the add-in from the list and click “Enable”
• Restart Outlook

Outlook 2007:

• Click on “Help->Disabled Items” menu command
• Select the add-in from the list and click “Enable”
• Restart Outlook

Outlook 2003:

• Click on “Help->About Microsoft Outlook” menu command
• Click the “Disabled Items” button in the lower right-hand corner
• Select the add-in from the list and click “Enable”
• Restart Outlook