Here's why:
* Missing a subject: We know a dog howled, but we don't know *which* dog. The sentence needs a subject (e.g., "The dog" or "My dog").
* Missing a verb: While "howled" is a verb, it's in the past tense, and the sentence needs a present tense verb to describe a continuous action happening all night.
To make it a complete sentence, you could add a subject and change the verb:
* "The dog howled loudly all night."
* "My dog was howling loudly all night."