PI published my work after I graduated without informing me (and with a different first author)
It is a difficult situation, and the course of action may depend on many things we do not know (e.g. the influence of the PI, your current situation and dependency from him – do you need letters of recommendation, or did you leave academia –, how much factual evidence you can provide for your part on the works, etc.).

Here are various approaches which seem possible, but any of them may be impractical or dangerous depending on the unknowns mentioned above.


Contact the PI to sort the situation out; it is better not to corner her, but to let her an honorable way out or probably nothing good will happen.
Contact the university or department to notify the misconduct. It might be better to only do that after contacting the PI, but it really depend on the history.
Contact an ethic committee if one exists in your field (e.g. an ethic committee for publications, including misattribution of authorship, has been set for maths by the EMS).
Contact the editor-in-chief of the journal the paper is published in, and notify her that you should be first author (providing evidence) and that you were not asked for permission to publish the work.


Be aware that any of the above move may turn out to be explosive; it is difficult to say more given the limited amount of information. So let me end with probably the most important advice:

Seek guidance by a senior researcher you know and can trust with your case. You will be able to discuss the specifics of your situation, and someone ready to back you up is of utmost importance in this kind of situation.