There have been some cases were the blueprints are disappearing. The don't actually disappear, they just become invisible.
This happens when you install a blueprint in a slot but don't have enough minerals. Players often do this to see mineral requirements. As there aren't enough minerals, no job is created, making it look like the blueprint has disappeared (it's in a slot with no job).
In this case there are 2 ways to get it back, cancel the lease of the slot or petition a GM to eject the blueprint. We have fixed the bug but blueprints installed before the bug fix are still in this invisible mode.