Updated from 11 to 12 ,
& Prolouge Synth is Missing ’ Which is a Synth i use regular ’ so
Rescanned all folders , But is still Missing ’
Any ideas ’ aside from A full Reinstall ??? ’ Arghhh ’ 
Prolouge is no longer supported in C12, as well as a few other older Cubase VSTis.
Others no longer supported due to technical reasons is Spector, Mystic and LoopMash.
Just did a new track with one instance from prologue. Would have been nice to know before the upgrade that they would be gone. All is good , just frustrating.
Have a look here - Roger Schumman provided the solution, if you have Prologue from an earlier version of Cubase.
Mystic, Prologue and Specter not in Cubase 12 - Cubase - Steinberg Forums