My ex wrote after months of silence. Why now?

A message after months opens contact again; it does not explain why that contact resumed. The wording may offer a reason, an apology or only a greeting. None of those automatically settles what ended between you.

Before reopening more of the conversation, you can ask what he wants to talk about. If contact was previously ruled out by either of you, that boundary still matters; a new message is not a reason to press past someone else's no.