Hey Tyler,
Thanks for reaching out.
If the Tribe__Events__Main class is not found, that indicates a problem with file versions on the site.
The only way to fix this would be to access your site’s plugins directly over FTP. You would need to go to /wp-content/plugins and delete the /the-events-calendar directory there, and then also the /events-calendar-pro directory there.
This will not delete event data.
If you are not familiar with doing this over FTP, then your web host may be able to do this upon request if you ask their support team to do so.
Once that both things are deleted, and there is no trace of either plugin on your site (again, this will not delete event data), download a fresh, brand new copy of The Events Calendar from here ? http://wordpress.org/plugins/the-events-calendar
Install this on your site.
Activate it. Does the error your reported above arise?
If not, then download a fresh copy of Events Calendar Pro from here ? http://theeventscalendar.com/my-account/downloads
Install this on your site.
Activate it. Does the error your reported above arise?
Let us know what you find by doing the specific process above!
Sincerely,
George