Outlook has a habit of disabling add-ins without asking for permission to do so. Depending on what version of Outlook you are using this can be corrected in a number of ways. For example, in Outlook 2013 and 2016, you can usually re-enable Bananatag by doing the following...
- Click on File
- Click on Info
- Select Manage Add-Ins or Slow and Disabled COM Add-ins
On the new screen that opens look for Bananatag and select "Always enable this add-in" if it is present.