Possible, yes - likely, no, at least not in the area where we find pyramids (I assume you're talking about the pyramids in Egypt, and not the pyramids in Somalia or in Central America). This is because the sand only builds up at a certain rate, so it is not likely that there would be large pyramids that could have been buried in that time.

Pyramids under water is less likely yet. We would know about them, because we could see them using sonar.