A subpoena is a judicial order to appear at a place at a given time. Failure to do what is judicially ordered is not a smart idea. But the penalty is not self-executing; it requires that the issuer of the subpoena follow up with obtaining (or issuing) a further order penalizing the party for the failure to appear.
If the party is a deponent under subpoena issued by an attorney, then usually the “penalty” is to have the court issue its own order that you appear at a deposition.
If the subpoena was directly issued by the court at the request of one of the parties, and you fail to appear, this may well result in an order that you show up before the judge and explain why you shouldn’t be fined or jailed for failing to appear.
A legislative subpoena is a different animal, enforcement of which starts with the legislative body. If need be, they can go to the court system to get orders enforcing the subpoenas.