In TNG episode "Yesterday's Enterprise" the Enterprise C (the last starship with that name before TNG's time) travels in time 22 years to the future (TNG's present) just before being destroyed by the Romulans.
This event causes an alteration of the timeline and in the present of TNG, there is now a long war between the Federation and the Klingons, a war the Federation is losing. There the Enterprise C meets the Enterprise D.
We know the fate of the Enterprise C in the original timeline (being destroyed) but i dont remember any reference from the episode to what happened to the Enterprise C in the alternate timeline. Did she just disappeared 22 years ago or returned to the federation? People in Enterprise D should know her fate.