First, you should ask a supervisor or manager who will actually know why you have been overlooked. Speaking to colleagues will not get you answers only rumours.
Be upfront about why you have not been promoted, without being confrontational. Any employee, even your manager, understands that promotion is a reasonable reward that every employee expects for the good work they put in.
Do not be rude or act entitled. Your manager or supervisor may not be the decision maker. Crude behaviour, rude language and swearing will only tell them that you were not right for promotion.