The Teams Outlook Add-in automatically gets disabled if it is deemed “slow” by Outlook. If disabled, it would look like this:  

The Add-in would just need to be reenabled to see the icon again: 

 
Troubleshooting Steps: 

1. Navigate to the top left and click on “File”  

2. On the bottom left click on “options” 

3. A window will appear. Click on “Add-ins” 

4. Navigate to the section labeled “Disabled Application Add-ins”. Click on “Microsoft Teams Meeting Addin…”. Click on the Drop Down on the “Manage” section and click select “Disabled Items”. Then press Go 

5. Click on “Addin…” to highlight it. Then Click on Enable: 

6. Another window will appear. Select the box with “Microsoft Teams Meeting…” and press OK. 

7. Reload Outlook and navigate to the Calendars section on the bottom left: 

8. The Outlook Add-in should appear