Back in May, webmail was disabled to fix the high number of 503 errors on Tommy. Most people don't use webmail, so having it enabled for every account was wasting resources. You can read more in the News article about it here: https://helionet.org/index/topic/64390-503-errors-update/
If you want to use webmail, you can re-enable it inside Plesk by going to:
Login > Continue to Plesk > Mail > Mail Settings > [Your Domain] > Webmail > Select Roundcube > Ok
Once you re-enable webmail, it will take up to 2 hours to get back up and running, since it needs an Apache restart.
Edited to add: For other users who may find this post from a forum search, webmail on Johnny and Tommy servers is disabled by default but can be enabled by the user with the steps above.