I got charged with simple assault and harassment it was ma and my girlfriend. She dont want me to go to jail she isnt showing up at court what happens

Depends on the facts of the case really.  The Commonwealth can compel her testify with a subpoena.  That means if she doesn't testify they can arrest her and hold her in contempt.  However, in some cases if the complainant doesn't show up the Commonwealth will dismiss the charges.  What county you are in makes a big difference.  Many counties don't even require the victim to testify at the Preliminary Hearing (they don't have to according to the rules) but a few do.  Your best way to answer this question is to talk to your attorney and find out how your county and judge handle these matters.   It is not unusual for the complaining witness to be uncooperative in these types of cases.  Most prosecutors have handled this situation a few hundred times and will have well defined rules and procedures.  The only way to know what those are is to have local counsel who will likely have handled a similar number of cases.