It depends on the Judge and/or the State Attorney involved with the case. Some may take no action at all, but others may issue a warrant for an Order to Show Cause. If they issue the warrant then the witness will have be brought in to explain to the Judge why they missed the hearing/trial. If the Judge is satisfied then the witness will most likely be told to come back for a new trial date. If the Judge isn't satisfied then the witness can be held for contempt of court.
I'm going to take a wild guess and suggest that you are the witness contemplating whether to appear or not. If you have been properly subpoenaed, you face potential contempt charges and could thereafter be fined and/or jailed.In addition, if you are a critical witness for either side, do NOT believe that by not appearing that the case will just go away. The Judge will most likely grant a continuance of the trial and grant whatever measures necessary ( including a material witness warrant) to secure your attendance at a later trial. It is much better to just cooperate. Contact the party who subpoenaed you and find out your status.