Nobody knows what will happen, but since the national government sees the whole referendum as illegal, chances are slim they will suddenly change their mind afterwards and grant independence. So from their point of view, nothing is supposed to happen, referendum or no referendum.
If the referendum yields a no, nothing will happen anyway, apart from the independence movement trying to find new ways to reach their goal, I guess.
If it yields a yes, nothing will happen initially either, because the national government won't accept the legality of the referendum.
Afterwards, however, things might happen, ranging from Catalan people just being very disgruntled to a full-out civil war / independence war. It seems this will mainly depend on how far the Catalunians are willing to go. If they declare independence without Spain's consent, I doubt that Spain will sit idle, but the real question is, are they willing to risk war?
If I were a betting man, I wouldn't put my money on Catalunia going that far, but hey, predicting the future is a tough job.