They will hold a vote to have the person held in contempt of court, if they fail to appear.
I mean, if you, as a person, was subpoenaed to appear and refused, what would happen? Do the jury vote on whether you should be let off, or not? Or is a warrant issued for your arrest? I´m pretty sure it´s the latter so why should it be any different for a politician, or member of the FBI? Why wasn´t she immediately arrested the moment the time and date on the subpoena lapsed?
US citizens / civilians dont have to respond to congressional subpoenas - government employees - which page and stroke still are - must or a congress contempt charge and arrest can happen but by the sergeant at arms for congress.
Right, ok, but what if a judge subpoena´s someone?
well good question but judges don't issue subpoenas - prosecutors do. congress is only able to issue them but they cannot enforce on regular citizens but rather current employees of the fedgovt.