Hi, it could be a combination of things. Your design might depend for e.g. on a hyperion shrinker with X amount of bonuses. it's caught me a few times when I have demolished one of my hyperion buildings to rebuild them in a better location and half my ship designs disappeared till it was rebuilt.
Other options are you might have designed it with a tech you traded for, or a component (resource) you have run out of.
The ship should still be visible in the designer, and you can investigate it to see if it is over mass or missing a resource or something like that.
