Having scored a large quartet I formatted and printed it out. All parts came out fine and still exist, but one of the parts disappeared from the score??? is there some way to get it back in the score, since the part is still in the system somehow?
Welcome to the forum, @inotmark!
In Setup mode, select your full score layout on the right, and make sure that the checkboxes for all of your players are ticked on the left.
(Or select the missing player on the left, and make sure the checkbox for the full score is ticked on the right.)