There have been reports of this happening in a very small amount of FareLock reservations littered throughout this forum that I've read in the past year or two. In all of them (at least that I recall), as long as your FareLock time is not expired, UA will work wonders to give you that fare that you rightly have, should you want it.
But, once that time is expired, no matter what has happened, they do not give you that fare (as they shouldn't).
As for the "what happened", no one knows
