A fine or jail time, same as in every other contempt case. In this particular matter, Mnuchin might actually have a leg to stand on, that's why it needs to go to court. This is a congressional fishing expedition, everyone involved knows they won't find 1099's from Putin, what they're looking for is something to use politically. Since most people have no concept of how tax law works, it's vary easy to make perfectly legal deductions appear sinister. The last bit that was released was a perfect example. It showed tremendous losses, and I only heard one single commentator explain how it was possible to have huge paper losses and still make a profit.
While the idea that there is income from criminal activity listed on Trumps 1040 is absurd, he should release the information because he said he would. That statement makes the entire issue Trumps creation, and it's the only compelling reason to demand they be released.