Does the US constitution state that nothing can be done retroactively?

The Constitution does not explicitly prohibit the application of laws retroactively, but it does contain several provisions that limit the government's ability to do so. For example, the Due Process Clause of the Fifth Amendment prohibits the government from depriving people of life, liberty, or property without due process of law. This means that the government must generally give people notice and an opportunity to be heard before taking away their rights. In some cases, the Due Process Clause may also prohibit the government from applying a new law to conduct that occurred before the law was enacted.

Another provision that limits the government's ability to apply laws retroactively is the Ex Post Facto Clause of Article I, Section 9. This clause prohibits the government from passing any ex post facto law, which is a law that retroactively makes an action illegal that was not illegal when it was committed. However, the Ex Post Facto Clause only applies to criminal laws, and it does not apply to civil laws, such as tax laws or regulatory laws.

Finally, the Contracts Clause of Article I, Section 10 prohibits the government from passing any law that impairs the obligation of contracts. This means that the government cannot retroactively change the terms of a contract, such as by reducing the amount of money that one party is obligated to pay.