What type of law punishes people for doing something that was once legal but is illegal?

It's called an "ex post facto" law. "Es post facto" is latin for "after the fact."

In the U.S., and most other developed nations, such laws are constitutionally prohibited, because they're viewed as manifestly unfair. Most would agree that it's absurd to punish a person for an act that was legal at the time the act was committed.

Accordingly, all laws must be prospective (applying forward from the time of enactment), rather than retroactive (applying backward from the time of enactment).