Topic title says it all. What would you do?
Then I would quit with Bitcoin because then I would have no reason to stay and keep using Bitcoin, I would go find something else to make money with.
I would not start from 0 again because that takes to long.
Yeah it would be hard to start from 0 again as it will require lot of efforts and the fact is that after loosing a big amount you would be disappointed and won't show any interest in earning it again.