Did the Track Email button disappear from your Gmail's compose box? If so, Bananatag may have been disabled and needs to be re-enabled. There are two places the extension may of been disabled: in your browser or in your inbox.
If you see the Bananatag "b" logo in the top right corner of your Gmail inbox please click on it and select Enable Bananatag. This should refresh your browser and you should see the Track Email button again.
The Bananatag "b" logo in your Gmail inbox.
If the Bananatag logo is not visible at all in your inbox, then the extension may be disabled in your browser.
To check for this in Google Chrome click...
-> Chrome menu (top right corner) -> "More Tools" -> "Extensions"
To check for this in Mozilla Firefox click...
-> Firefox menu (top right corner) -> "Add-ons" -> "Extensions"
When you are done re-enabling the Bananatag extension please close your internet browser completely, and then open it again, before returning to your Gmail inbox.