What happens after SC?

I don't know. But it is not looking good. Weak 3rd or even a 4th placebehind a non-campaigning Romney in SC is not a good thing From Powerline:

Fred Thompson just spoke to his supporters in South Carolina, where he appears headed for a weak third or fourth place finish. Thompson thanked everyone and then spoke about America and conservatism's contribution to it. He did not talk about Florida or being in the race to stay.
Word is that Thompson will return to Tennessee to visit his mother in the hospital. While he's there, presumably, he'll consider his options. Frankly, and I say this with regret, it's difficult to see the case for him remaining in the race.

Courtesy of Powerline