https://www.lawfareblog.com/what-can-congress-do-if-flynn-or-anyone-else-refuses-comply-subpoena

Just as the Supreme Court has long recognized congressional subpoena power, so too has it upheld the enforcement power of contempt. If a witness refuses to comply with a subpoena to either produce documents or testify, Congress has a few options: inherent contempt authority, the criminal contempt statute, and various civil enforcement mechanisms.

How the Committee proceeds may depend both on how much cooperation it anticipates from the Department of Justice ... and on its own calculation about the wisdom of forcing the issue. Congress has tools at its disposal, but how it uses them is a strategic calculation.