Exactly as your other comment finds, monarchies can be dictatorships if the monarch really takes control. The typical method of control tends to be different though, with monarcies often using traditional power (such as heredity and religion) and dictatorships usually being the result of military force (coups and conquerings).

I think the more useable difference is that a monarchy is a system ruled by a single person, while a dictatorship is a single person ruling. It depends on whether we’re talking about an existing system of hierarchy or a person at the top; a leader vs, well, a dictator.

It’s not common, but I’d bet a dictatorship that lasts long enough will become a monarchy or institue some form of oligarchy, while a monarchy could become a dictatorship without changing much; maybe reverting with the dictators passing or simply collapsing.