What happens if you don't celebrate Passover?
Orthodox Jews follow Jewish Law, so the punishment given for those Jews that eat Chametz (leavened bread) on Passover is Kaarais. Although there are numerous explanations of what that means, it is very bad, regardless of how you understand it. The simple understanding is that persons soul will be cut off. What that means is not simple - however.
For non-Orthodox Jews, the punishment is that by cutting yourself off from the Jewish people, denying your heritage, you deny yourself of a rich and rewarding experience that enhances your life. Many non-Orthodox Jews also accept the idea of making yourself holy, and missing out on Jewish occasions is a missed opportunity to be holy.