Unable to delete calendar invitation

Hi,

My iPad calendar is synced to outlook. I can view,accept,delete,decline any of the calendar invitations. However, I have an invitation that I'm unable to delete. The due date has already passed but it still keeps showing up as new invitation and when I click on it, I can't take any action on the event. How can I remove the event ? I would really appreciate it if anyone can help me with this.

Thanks,
Seema