She heard a gunshot. Then the fell silent. how would you improve this to make it sound better?
That very much depends on the context of the scene, style of the writing, and so on. In some works, what works best may be a detailed description like DWKraus wrote. In others, what provides the best effect may be:
Just this, as a whole paragraph, or even section.
There are lots of different possibilities for lots of different stories.
If your desire is to expand the description of the sound, it may be helpful to remember that the sound of a real gunshot is quite different from the exaggerated movie soundtrack sound. Rather than a deep "boom", most real-world gunshots have more of a "pop" sound. Some smaller calibers have a sound better described as a "crack".
The resulting sound also depends on distance and whether the gun is fired indoors or outdoors. Indoors, the reverberation of a gunshot can make it sound deeper and much louder. Outdoors, nearby gunshots have a sharper sound. The further away the gunshot, the less high-frequency energy the sound has. This makes distant gunshots sound deeper.
Finally, it's easy to forget that gunshots are extremely LOUD. If a gun fires near you and catches you off-guard, your breath catches in your lungs, your heart all but stops, and your ears will complain of the mistreatment for a long time.
Add Detail and Emotional content:
While functional, this statement is rather thin. A gunshot should be a jarring thing in a story. It often represents someone or thing dying. So more description would add intensity and punctuate the event. The gunshot transforms the world.
I would involve more senses into the event and extend the duration. Maybe even leave the nature open until she identifies in her mind that it was a shot. Use words emphasizing the traumatic nature of the event. Context is critical here, but for example:
"Her eyes barely registered the flicker of light before the the sound struck her like a slap. The noise was shocking and overwhelmed her hearing. Afterwards, her ears rung with the echo. The burning smell of powder wafted through the air. In the sudden silence following, she froze until remembering to breathe."