A subpoena is a writ summoning you to court. It is not an invitation.

If you do not appear at the time, date, and place specified in the subpoena (for a trial or a deposition), what may (not "will") happen is "the state" (the local prosecutor's office), if you were/are a state witness, or the defense (attorney for the defendant), if you were/are a defense witness, asking for an "order to show cause," which is a hearing. At such a hearing, you would explain to the judge presiding over the case why you failed to appear for the trial.

An order to show cause hearing is practically never set if the case has not ended. Sometimes one is set for a case that has ended. What type of case would one side or the other request an order to show cause hearing? Typically a serious case and/or one where the witness who failed to appear was integral to the state (or defense) prevailing at trial and did not because of the absence. In a petit theft case? Likely no. In a murder case where the witness' testimony would be needed for a finding of guilt (or a "not guilty" verdict)? Likely yes.

In an order to show cause hearing, if the witness' explanation for not appearing is unsatisfactory to the judge, the absent-at-trial witness would be held in "contempt of court" and ordered to do community service hours, spend days in jail, or something else.

If you fail to appear for an order to show cause hearing (also known as a "rule to show cause" hearing), a warrant would be issued for your arrest, which would be a foolish route to take when offered such a choice - appear or don't appear. Definitely show.

A subpoena mandates your appearance at a scheduled proceeding. Again, it is not an invitation. However, sometimes nothing is done even if you fail to appear.

Sometimes.