What happens if a witness is subpoena and don't show up for court?

A subpoena to appear to testify is a court order. If you disobey the subpoena by failing to appear, you will be held in contempt, and the court will likely issue a bench warrant for you, and you will be arrested. You are better served by appearing in court in response to the subpoena. If you are called to testify, but believe your testimony is likely to incriminate you, then you inform the court that you are invoking your Fifth Amendment privilege not to testify rights and refuse to answer the question on the grounds that your testimony might incriminate you. You cannot be forced to testify against yourself. If you do not appear, however, you will be arrested (probably) for failing to comply with the subpoena.