Is this a correct sentence sudden flash of light in the night sky.?

That's not a complete sentence. It's a phrase describing a sudden flash of light in the night sky.

To make it a sentence, you need to add a subject and a verb. Here are a few examples:

* There was a sudden flash of light in the night sky.

* A sudden flash of light illuminated the night sky.

* We saw a sudden flash of light in the night sky.