Hi,
Sorry for the issues here. I can work with you to resolve this.
If you updated The Events Calendar to 4.0 and leave Community Events at 3.12.x or earlier it will break the site. If you cannot update Community Events for whatever reason you should remain on 3.12.6 for the Events Calendar and follow the steps below to set that up.
Unfortunately, the only way to fix it is to access the files of you site and delete the older version of Community Events.
This can be done by either your hosting account file manager or through ftp. The files will be found here:
wp-content/plugins/the-events-calendar-community-events/
Please make sure to remove all copies of Community Events.
That will bring your site back and then you can download the latest version of Community Events (4.0) from your account here and manually upload it to your site.
We also have this guide for manual updates:
Manual Updates
If you need your site right away with community events I recommended downgrading to the 3.12.x for all the plugins.
You can download the older versions from your account here on theeventscalendar.com and older versions of the core plugin here: https://wordpress.org/plugins/the-events-calendar/developers/
And follow these steps for manual updates:
Manual Updates
If you have issues please let me know and I can help out.
Thanks