Try deleting and then re-adding that event. It might be some corrupted information.
If the calendar is being synced with an online service like iCloud or Google, you can also try turning off the syncing in Settings > Mail, Contacts, Calendars: your account. Tell it to delete the data from the iPad when prompted, Open the Calendar app to make sure it's gone, Then turn it back on in settings.
This will clear and re-sync all the calendar data. It often fixes problematic events and issues. You wont' lose any data, as long as it is stored correctly online.
One other thing I've seen that sometimes causes problems. The iPad only supports two or three alerts per event. The web interface can usually add more. I've seen Calendar choke on this when you try to edit the event. It's been a couple years since I ran into this, so I don't really know if it's still a thing.
My workaround for this was to create more than one event for things that needed that much lead time. A "Remind Me About" event for the earliest alert, and then the actual event later.