Most direct translation:
* Kei hea a Nana? (pronounced kay hea a nah-nah) This literally translates to "Where is Nana?" and is the most direct way to ask.
More informal:
* Nō hea a Nana? (pronounced no hea a nah-nah) This literally translates to "Where is Nana from?" but can also be used to ask "Where is Nana?" in an informal setting.
With a little more formality:
* Ahea a Nana? (pronounced ah-hea a nah-nah) This literally translates to "Where is Nana now?" It implies that you're looking for Nana at a specific point in time.
Using the word for grandmother:
* Kei hea te kuia? (pronounced kay hea te koo-yah) This means "Where is the grandmother?" and can be used to ask about Nana specifically if you know that's who "te kuia" refers to in that context.
The best choice will depend on the specific situation and your relationship with the person you are asking.