From the summary of Congressional Subpoenas: Enforcing Executive Branch Compliance (R45653, Mar. 27, 2019):
Congress currently employs an ad hoc combination of methods to combat non-compliance with subpoenas. The two predominant methods rely on the authority and participation of another branch of government. First, the criminal contempt statute permits a single house of Congress to certify a contempt citation to the executive branch for the criminal prosecution of an individual who has willfully refused to comply with a committee subpoena. Once the contempt citation is received, any prosecution lies within the control of the executive branch. Second, Congress may try to enforce a subpoena by seeking a civil judgment declaring that the recipient is legally obligated to comply. This process of civil enforcement relies on the help of the courts to enforce congressional demands.