The sim is completely frozen, but I can still move and do everything the client can do save for viewing replays. My last action was to order about 35 to 50 orbital fabbers off a moon as it was changing orbit / smashing. I tried closing and reopening the client, but after reconnecting I'm stuck in a loading screen without the world loading being displayed.
Yes, happens from time to time, but I was unable to find out what actually caused it. If you wait long enough you might get the message that the connection to the server was lost.