Drive missing in Disk utility
Either the SSD, or its connecting cable, or the underlying Mac Mini (either hardware or software) has a problem. 

Isolate the issue by taking the SSD out of the Mini, and connecting it to another Mac. If the drive mounts, that suggests the connecting cable or the underlying Mac Mini is the problem. Keep swapping parts until you find the bad link in the chain. 

Cables are fragile. If there are now two drives in the Mini and the other drive works, try swapping the two drives' cables to see if the SSD's cable is the problem.