I know there have been posts on here recently about flights missing from people's upcoming "my trips." This is NOT quite that.

so yesterday I bought YYZ-FCO on(3/10/16) and then FCO-ATL-YYZ on (3/15/16) I made my purchase, ticket was issued, got seats. all good. Later in the day, a friend decided she wants to join me and wants to mirror the same itinerary. The FCO-ATL flight now doesn't exist. When I booked yesterday there were PLENTY of seats for sale so I can't imagine it's sold out in a matter of hours. So I called DL and the rep said she can't even pull up that FCO-ATL flight on her end but confirmed that I'm booked in it for 3/15. The flight exists for 3/13, 3/14 and 3/16, etc. so why would that flight now all of a sudden not be showing up just for that day (3/15)? truly puzzled and would appreciate any insight.. Thanks.