Refusal to respond to a subpoena will result in you being found in contempt of Congress. Congress then has several options, ranging from referring you to the DOJ for prosecution to referring the citation to a federal court, effectively converting the subpoena/contempt into contempt of court, or literally imprisoning you in the House basement, which will never happen but it's amusing to note that they technically have that authority.

Subpoenas of administration officials complicate this because the primary vehicle Congress has for enforcing contempt citations is a referral to the DOJ, which is part of the administration; a civil referral is, I think? its own kind of civil suit, which is time-consuming and complicated, and it's likely that the House majority will keep their powder dry on referrals about administration officials until they have a really good case.

But random civilians aren't so fortunate, and can effectively be compelled to testify, and it's likely that Congress would not have much trouble getting their subpoena enforced.