Most likely scenario IMO: You were advised of a schedule change via email. Either you missed that email (for instance, because it landed in your spam folder) or you didn't miss it but didn't take action. Usually, those emails provide you with an alternative itinerary and advise you to either accept it or call in for a change. If you take no action, eventually they take out your reservation and free the space for somebody else. If that is indeed the case, your ticket is still good but you will need to find flights with available seats.

Again, no guarantees this is what happened. But I saw the above happening a few times with family & friends which call on me with all kinds of airline trouble.