That is the $64K question. No one really knows, because it's never come to that, yet. There are competing legal schools of thought. One camp (the camp I'm in) argues that since states are sovereign entities, they could just default on their debt.
Anther school of thought, which I believe to be a stretch, describes some kind of federal receivership. But, states, like the federal government are sovereign entities. There is no constitutional provision for one sovereign (the feds in this instance) to take control of another sovereign (the state).
A GM style bailout, and probably the third school of thought, wouldn't be out of the question, either; Some kind of pre-arranged loan where the feds dictate certain terms to the state, leaving the state the option to acquiesce, or pass.
It really is uncharted water, legally. What would actually happen is anyone's guess.
Well...
Since the left loves to tell us how much money California contributes to the federal Government, I’d suggest succession.
Let California become the liberal Utopiaa they want it to be, and see how fast we can build border fences to control the illegal immigration when they figure out what a “liberal Utopia” really is.