A subpoena is an order of the court to appear (even if signed by an attorney). Failure to comply could result in anything from a finding of contempt to arrest.
You can be arrested. You have to testify because you know something and they want to find out what you know. They are the court it's not like you told your friend that you would be coming over and didn't.
If you are subpoenaed, which is when the court requires to appear for any number of reasons, and you fail to appear the court may put out an arrest warrant and have you brought to court.
