Strange indeed , because usually you have a day or so before the State rests its case which allows for time to summons the witness. If the witness was not a hostile witness you could have called them and served them with a subpoena at the Courthouse. You may be able to withdraw your plea but you will have to communicate that clearly to your lawyer.
It is difficult to undo a voluntary plea of guilty, but it can sometimes be done. You need to consult an appellate attorney. That person can review your case and advise you from there.
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It can be done, but it can be tougher to do when you pled voluntarily. The judge would have asked you if you were pleading guilty of your own free will, that it was not coerced, and that you understood the consequences of your actions. The purpose of them asking these questions at your plea is to help avoid situations just like this. Find a good criminal appellate attorney right here on this site to assist you.