Why MailPoet Disappeared from my WordPress Dashboard?
In certain circumstances, if MailPoet detects problems with itself, it may be automatically disabled by itself and you won't see it on your WordPress dashboard.
If you try to re-install it via the Plugins page and see the error message Installation failed: Destination folder already exists., please try to re-install the plugin via FTP.
Don't worry, you won't lose any data from the plugin.