Object not is database

I have created some reportable items in the timesheet a month ago without putting any start or finish date. But now I am trying to mark it complete and the error message says "Object not exist in database". I seriously don't know what that means. If it means the reportable item as an object then it exists definitely and I am trying to mark it as complete. I cannot delete or complete it. I am seeing heaps of items in my timesheet and don't know how to get rid of this.

PLEASE PLEASE HELP!