If your phone is not charged, it won’t work. But your phone is charged, so it will work.

If your phone is not charged, it won’t work. But your phone is charged, so it will work. (Is this statement a fallacy)

This is a fallacy called Denying the Antecedent. At first glance it seems to make sense, because the consequent (whether your phone works) is directly related to the antecedent (whether your phone is charged) but it ignores other possibilities and confuses the directionality of the relationship.

For example, your phone could be charged, but have some other technical fault that means it won't work.

Another example would be "If it is raining, the sidewalk will be wet. It isn't raining, so the sidewalk is dry". This ignores the possibilities that it might have been raining earlier and the sidewalk hasn't dried yet; a car might have driven through a puddle and splashed the sidewalk; the sidewalk might be next to a lawn with a sprinkler that's been on; it might have been snowing and the snow has melted; someone might have spilled a drink on the sidewalk; and many other options that would result in the sidewalk being wet without the need for current rainfall.