My daughter is 6 bit food and her top tooth which was loose turned grey?

Time to fall out?: The age of 6 is about when the front teeth tend to loosen and fall out, as the permanent teeth work their way into the mouth and erupt. Whether that's the case and everything is normal or there is some specific problem, only a dentist who examines your daughter can determine. Have her dentist take a look to be sure.

Bite: Your daughter may have bitten into something that loosened her tooth further and damaged the nerve to the tooth.As a result the tooth is no longer viable and turned gray. Consult your dentist for further advice.

It's fine: Sounds like a baby tooth that will fall out very soon. It will not affect the adult tooth erupting from above. I usually recommend my patients eat an apple, then the baby tooth will fall out completely.

Trauma: Baby teeth often turn grey after a traumatic incident. The "nerve" of the tooth is damaged, blood leaks into the nerve chamber, and turns into a black-and-blue mark inside the tooth. Probably not of serious consequence, but have your dentist check to make sure there's no damage to the underlying developing permanent tooth.