First, make sure you are looking at this year's event. It is very common to open a past year's event.
After verifying you're on the correct year, if the event is displaying 'Sold Out' there are no more tickets available for the event. If the event is displaying 'ticket sales have ended' most events will still have ticket available for purchase at the door. Reach out to the organization hosting the event to inquire about ticket availability. Contact information for an event organizer is under the map on an event's web page.